Add Android Oboe backend and AMY wire socket transport

[linuxificator]

Summary

Adds a generic Android backend for AMY:

• native Android AAR service in a separate, unexported :amy process;
• Oboe/AAudio low-latency stereo signed-16-bit output at AMY's 48 kHz / 128-frame profile;
• private AF_UNIX / SOCK_SEQPACKET transport at <filesDir>/amy.sock;
• one unchanged AMY wire request per packet;
• same-UID peer validation with SO_PEERCRED, socket mode 0600, and a bounded 64-packet queue;
• minimal transport-only Java hello-world client that plays C4 through C5 entirely through AMY wire messages over the socket.

The client contract is framework-independent. The hello-world application does not import or call AmyService, does not start or stop AMY, does not load a JNI AMY client, and does not compile AMY source. It uses Android's public LocalSocket(SOCKET_SEQPACKET) API and sends raw AMY wire packets only. The AAR owns Android service startup through its private manifest ContentProvider, so the same client boundary can be used from Java/Kotlin, Godot, Qt, or another framework/language that can package the AAR and open the Unix-domain socket.

The AAR intentionally remains packaged into the same Android application/UID as the client because amy.sock is app-private (0600) and accepted peers are validated with SO_PEERCRED. That packaging requirement is separate from the client API: application/framework code has no AMY headers, source, JNI binding, or lifecycle call.

Realtime design

• The socket receiver thread never calls AMY directly.
• The audio owner drains already-queued packets at AMY block boundaries.
• The Oboe callback adapts arbitrary Android callback numFrames values to AMY's 128-frame blocks without adding another full-block output ring.
• amy.sock is published only after Oboe has started and its first realtime callback has executed, so a successful socket connection is also the engine-readiness boundary.
• Test-only audio capture preallocates its buffers before audio starts; the realtime callback only copies into those buffers and never performs file I/O or takes the writer mutex.

Client boundary

application/framework code
        |
        | ordinary AMY wire packets only
        v
<filesDir>/amy.sock  (AF_UNIX / SOCK_SEQPACKET)
        |
        v
independent Android :amy process -> AMY -> Oboe/AAudio

The Java example performs no service control. On launch it retries the private socket until it exists, then sends v0w0V10.0Z followed by the C-major note-on/note-off wire packets. The previous JNI socket wrapper (libamy_hello_client.so) has been removed entirely.

Validation

The contribution includes focused transport tests and Android integration/audio-level CI:

• bash tests/run_amy_unix_socket_test.sh validates packet round-trip, peer/ownership behavior, oversize handling, cleanup, and stale-path safeguards;
• Android CI builds the AAR and hello-world APK for arm64-v8a and x86_64 service binaries;
• CI explicitly verifies that the hello-world APK contains no libamy_hello_client.so and that the hello-world module has no native src/main/cpp client code;
• an Android 35 emulator performs two independent uninstall/install/cold-launch cycles and requires AMY/Oboe startup, successful pure-Java connection to amy.sock, an output-route diagnostic, V10.0, all eight C-scale note-on packets (60, 62, 64, 65, 67, 69, 71, 72), successful completion, and no socket failure;
• CI, rather than the example application, arms the optional test-only audio capture before launch, keeping the example strictly transport-only;
• the integration test captures four seconds of the raw signed-16-bit AMY render stream and the exact signed-16-bit callback buffer handed to Oboe, measures peak/RMS dBFS, requires byte-for-byte AMY-to-Oboe identity, requires at least -6 dBFS peak, rejects full-scale clipping, and uploads the WAVs/statistics as a CI artifact;
• validation run 32651283791 on corrected head commit a12c19bfdd9936dcb10ab8b8e39214041dd56cd9 completed successfully: peak -2.721 dBFS, RMS about -7.27 dBFS, zero clipped samples, and zero AMY-to-Oboe sample differences.

The audio-level regression previously caught a hello-world configuration error: AMY's V control is a 0..10 bus/master scale and the final mixer applies a 0.1 factor. The earlier example's V2.0 therefore selected only 20% linear master gain (about -14 dB relative to V10.0). The example and documentation use V10.0.

Scope

This PR remains deliberately self-contained and does not modify existing AMY core synthesis source files. The Android service contains the AMY implementation; application/framework code communicates with it only through the wire socket.

The development history is five reviewable commits, with the final commit specifically decoupling the Java hello-world from the AMY service API/JNI client and moving Android startup ownership into the AAR.