Prepare for open-source release under GPL-3.0-or-later; docs & test maintenance

- Add GPL-3.0 LICENSE and SPDX headers (GPL-3.0-or-later) to every source file
  (58 Kotlin, the C++ file, and the sample-pack HTML tool).
- README: state the GPL-3.0-or-later license (+ badge); fix stale "256 beats"
  -> "256 bars"; fix build instructions (the Gradle wrapper is committed);
  mention master recording and the sample-pack tool.
- DEVELOPER_HANDOVER: resync with the codebase — corrected the wrapper note,
  architecture diagram, full file map, the add-instrument/effect guide, Oboe
  (now default with fallback, not an opt-in toggle), moved SF2/XI to implemented,
  and added recently shipped features with an honest remaining/optional list.
- Fix a pre-existing broken unit test: SngFormatTest called the old
  SngFormat.import(text); the API is importInto(project, text). Tests compile
  and pass again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/cpp/native_audio.cpp

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // Native Oboe (AAudio) output backend for Sizzletracker.
 //
 // Opens a low-latency mono float Oboe output stream. On each real-time data

app/src/main/java/com/reactorcoremeltdown/sizzletracker/MainActivity.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker
 
 import android.Manifest

app/src/main/java/com/reactorcoremeltdown/sizzletracker/SizzleApp.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker
 
 import android.app.Application

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/AmbienceReverb.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import com.reactorcoremeltdown.sizzletracker.model.AmbiencePresets

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/AudioEngine.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import android.media.AudioAttributes

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/Effects.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import com.reactorcoremeltdown.sizzletracker.model.DelayDivisions

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/MasterRecorder.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import java.io.File

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/NativeAudioBridge.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/SampleRecorder.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import android.annotation.SuppressLint

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/SampleStore.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import java.nio.ByteBuffer

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/SampleVoice.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import kotlin.math.exp

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/SoundFontLoader.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 import java.nio.ByteBuffer

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/SynthVoice.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/Transport.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.audio
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/BindingStore.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 import android.content.Context

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/GamepadInput.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 import android.view.InputDevice

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/InputAction.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/InputActions.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/InputRouter.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 import kotlinx.coroutines.flow.MutableSharedFlow

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/KeyboardInput.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 import android.view.KeyEvent

app/src/main/java/com/reactorcoremeltdown/sizzletracker/input/MidiInput.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.input
 
 import android.content.Context

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/PresetIo.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.model.ToolboxSlot

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/PresetLibrary.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.audio.SampleStore

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/ProjectIo.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.model.Cell

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/ProjectStore.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.model.Project

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/SettingsStore.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import android.content.Context

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/SngFormat.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.model.Arrangement

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/SongLibrary.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import java.io.File

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/ThemeIo.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import androidx.compose.ui.graphics.Color

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/ThemeLibrary.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.io
 
 import com.reactorcoremeltdown.sizzletracker.ui.theme.RetroPalette

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/AmbiencePresets.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Arrangement.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Mixer.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Music.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Pattern.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Project.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/model/Toolbox.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.model
 
 /**

app/src/main/java/com/reactorcoremeltdown/sizzletracker/playback/EnginePlayer.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.playback
 
 import android.os.Looper

app/src/main/java/com/reactorcoremeltdown/sizzletracker/playback/PlaybackService.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.playback
 
 import android.content.Context

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/App.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui
 
 import androidx.activity.compose.BackHandler

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/AppViewModel.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui
 
 import androidx.compose.runtime.getValue

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/Responsive.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui
 
 import android.content.res.Configuration

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/StackedKeyboard.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui
 
 import androidx.compose.foundation.background

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/components/GlyphCache.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.components
 
 import androidx.compose.ui.text.TextLayoutResult

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/components/Piano.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.components
 
 import androidx.compose.foundation.background

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/components/Widgets.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.components
 
 import androidx.compose.foundation.background

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/mixer/MixerScreen.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.mixer
 
 import androidx.activity.compose.rememberLauncherForActivityResult

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/settings/SettingsScreen.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.settings
 
 import android.content.Context

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/theme/Theme.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.theme
 
 import android.app.Activity

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/DelayEditor.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import androidx.compose.foundation.Canvas

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/LfoEditor.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import androidx.compose.foundation.layout.Arrangement

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/ParamControl.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import androidx.compose.foundation.layout.Column

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/SamplerEditor.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import androidx.activity.compose.rememberLauncherForActivityResult

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/SoundFontEditor.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import androidx.activity.compose.rememberLauncherForActivityResult

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/toolbox/ToolboxScreen.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.toolbox
 
 import android.content.Intent

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/tracker/ArrangementRoll.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.tracker
 
 import androidx.compose.foundation.Canvas

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/tracker/PatternGrid.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.tracker
 
 import androidx.compose.foundation.Canvas

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/tracker/TrackerScreen.kt

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package com.reactorcoremeltdown.sizzletracker.ui.tracker
 
 import androidx.compose.foundation.background