Add Godot Android service integration

[linuxificator]

Dependency: stacked on #1136

This PR depends on #1136 (Add Android Oboe backend and AMY wire socket transport) and should not be merged before #1136 is accepted.

The branch is built directly on the current #1136 head, a12c19bfdd9936dcb10ab8b8e39214041dd56cd9. The Godot contribution itself is exactly three commits on top of that head. Until #1136 is merged into main, GitHub will necessarily show the #1136 commits in this PR as well because a Shorepine PR cannot use a branch from the fork as its base.

For the incremental Godot review, the relevant stack is:

1. 63f07daa — Add Android transport backend to Godot Amy API
2. 0f100c27 — Add Godot Android hello-world and CI
3. f77cf938 — Document Godot Android service integration

Summary

Adds Android support to the existing high-level Godot Amy.gd API without running or compiling a second AMY engine in the Godot process.

On Android the path is:

Godot game code
    |
    | Amy.send(Dictionary)
    v
Amy.gd
    |
    | shared Dictionary -> AMY wire encoder
    v
AmyClient (pure Java transport helper)
    |
    | LocalSocket / SOCK_SEQPACKET
    v
<filesDir>/amy.sock
    |
    v
independent Android :amy service -> AMY -> Oboe/AAudio

The same message(Dictionary) wire generator used by the existing Godot API remains authoritative. Android only changes the transport/backend after a wire message has been built.

Android client boundary

The Godot application:

• does not start or stop AmyService;
• does not load the AmySynth GDExtension on Android;
• does not compile/package AMY C/C++ source in the Godot process;
• does not use a JNI/native socket client library;
• packages the prebuilt amy-service AAR in the same APK/UID and sends wire messages through its private socket.

AmyClient is a small pure-Java adapter around Android's public LocalSocket(SOCKET_SEQPACKET) API. It contains no synth/DSP code and owns no service lifecycle.

The existing desktop GDExtension and web/WASM backends remain unchanged in architecture.

Godot example and packaging

Adds godot/android-hello-world, which demonstrates:

• the normal Amy.send(Dictionary) API rather than direct test-only socket calls;
• an Android export plugin that packages the service AAR;
• ARM64 device and x86_64 CI export presets;
• deliberate exclusion of amy.gdextension, godot/bin, AMY source, and any second AMY native client library.

The source-tree prepare.sh builds/stages debug and release AARs for development/CI. Downstream Godot applications can package a prebuilt AAR instead; they do not need to rebuild AMY as part of every Godot project.

Validation

The cleaned three-commit stack was validated in a fork-local PR directly against the exact #1136 head, so the test diff contained only these Godot changes.

Validated code head before the documentation-only startup clarification: 2f1522b5cc772bb3093068f2e298a64627f5bbec. Current head: f77cf9389a800a69396170f502f8c6b380ca2329.

Successful validation runs:

• Godot Android run 32659597021: all build/import/export/packaging checks passed; emulator connected Amy.gd to amy.sock, sent Dictionary-generated AMY wire, and the captured AMY render/Oboe audio analyzer passed.
• Android AMY run 32659597044: socket transport, AAR/hello-world build, Android emulator end-to-end test, and AMY/Oboe audio analysis all passed.

The Godot emulator test is intentionally scoped to the AMY integration contract rather than Godot's renderer. The x86_64 preset omits the optional Canvas UI, and unrelated Godot/SwiftShader renderer failures are not authoritative once the AMY path is established. The test still requires:

• the separate :amy service and Oboe backend to start;
• Amy.gd to connect to the private socket;
• a high-level Dictionary call to generate and send AMY wire (including a note-on);
• no AMY socket/send exceptions;
• non-empty captured AMY-render and Oboe WAVs;
• the existing Android audio analyzer to accept the captured audio.

Documentation

Updates docs/godot.md, android/README.md, and the Android Godot example documentation to make the platform boundary explicit, including:

• safe backend_ready/backend_error handling;
• prebuilt-AAR packaging and same-UID private socket requirements;
• Android-supported wire-oriented GDScript API;
• desktop-only direct C-API helpers and pre-start engine configuration limitations;
• explicit instruction not to package the desktop AmySynth GDExtension in Android exports;
• the Android AMY startup sequence: AmyAutoStartProvider requests the separate :amy service, the native service publishes amy.sock only after Oboe has delivered its first realtime callback, and Amy.gd treats a missing socket as expected during startup and retries for approximately 10 seconds before emitting backend_error.

This PR is intentionally left as Draft while #1136 is reviewed/accepted.