feat: add experimental Android live-audio source

[matveyb9]

Summary

Implements the first experimental Android API 26+ foreground live-audio source slice. It adds no Android compatibility, device-support, physical-route or delivery claim.

Included

• Rust-owned Acoustic-1 PCM16 LE conversion and bounded Flutter Rust Bridge methods for text live PCM only.
• Regenerated committed FRB bindings and typed handwritten Flutter facade mapping.
• Android Kotlin AudioTrack / AudioRecord MethodChannel + capture EventChannel host with contextual RECORD_AUDIO, v1 format validation, playback focus, stop/release on interruption/lifecycle stop, and no route picker/background/retention behavior.
• Flutter Android adapter plus explicit experimental foreground playback/capture controls. Captured frames remain transient and decode only through Rust after user stop.
• Rust, Flutter MethodChannel, portable widget and adapter contract coverage.
• Bilingual implementation documentation, RFC/status updates and public pages that retain the no-build/no-device/no-route-support boundary.

Explicit non-claims

• This PR does not establish a reproducible Android target build, Android runtime behavior, granted microphone permission, focus/route observation, playback output, capture decode through a physical channel, supported device, Bluetooth/cable/radio route, background use, duplex or file live transfer.
• The sandbox has no Android SDK, so flutter build apk --debug could not run. This limitation is documented; it is not treated as successful target validation.
• The Android speaker-to-microphone acceptance template remains unexecuted. A reviewed physical-route report is required before any compatibility wording changes.

Validation completed

cargo fmt --all -- --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace
flutter analyze
flutter test                         # 13 passed, 2 opt-in native tests skipped
flutter build web --release
flutter build linux --debug
node tools/device-acceptance/validate-report.mjs tools/device-acceptance/fixtures/report-template-v1.json
node tools/device-acceptance/validate-report.mjs tools/device-acceptance/fixtures/android-speaker-microphone-template-v1.json
node tools/device-acceptance/test-validator.mjs
git diff --check

The web build retains its existing flutter_rust_bridge wasm dry-run warning; it is non-fatal and native Rust bridge support remains unavailable on web.