Context

Buzz mobile did not have an offline, on-device voice path. This adds Pocket TTS using the canonical April runtime from #3361 and fixes the mobile model contract to the April INT8 artifact set.

What changed

• Add a minimal mobile C ABI over the canonical Pocket TTS runtime, with a warm engine, engine-aware text chunking, explicit cancellation reset, and sticky cancellation.
• Share text preparation and PCM shaping between desktop and mobile while preserving the runtime's model-aware chunk boundaries.
• Download and validate the fixed April INT8 model set (165,890,159 bytes total) into a versioned cache.
• Add iOS audio-session handling and Android audio-focus/playback handling, including native playback-started events.
• Add the Flutter isolate worker, controller, conversation button, settings state, cancellation behavior, and no-repeat/excluded-channel safeguards.
• Build the native archive from the official sherpa-onnx artifacts; the Android child build installs missing Rust targets before compilation.

The branch intentionally contains no FP32 or multi-precision selection path, no duplicate sherpa extraction/package/sys layer, and no bundled model or native binary.

Validation

• cargo fmt --all -- --check
• cargo clippy -p buzz-voice --all-targets -- -D warnings
• cargo clippy -p buzz-voice-mobile --all-targets -- -D warnings
• cargo test -p buzz-voice --lib (35 passed, 4 ignored)
• cargo test -p buzz-voice-mobile --lib with the canonical sherpa archive (2 passed)
• cargo check --manifest-path desktop/src-tauri/Cargo.toml
• just mobile-check
• just mobile-test (896 passed, 1 skipped)
• Targeted Pocket voice Flutter tests (74 passed)
• iOS device and simulator native archives built; required C ABI symbols verified
• Android arm64 and x86_64 native libraries built
• :app:testDebugUnitTest passed with Java 21

Review notes

• The model manifest is pinned to upstream revision 58a6d00cf13d239b6748cb0769f35c580a8f606c, precision int8, and total download size 165890159.
• The settings golden now reports the fixed download as 165 MB.
• This remains a draft while hardware integration coverage and reviewer feedback are completed.