docs: define Android live-audio route design

[matveyb9]

Summary

This design-only slice selects Android API 26+ foreground playback/capture as AudioModem's first experimental live-audio platform target. It does not implement an Android adapter or enable any route.

It adds bilingual research comparing Android’s explicit permission/focus/PCM lifecycle with Linux PipeWire session/routing scope, plus a bilingual Android foreground live-audio RFC. The RFC fixes a narrow default-route design: 48 kHz mono PCM16 LE must be verified after native creation; microphone permission, playback focus, route changes, interruption and stop/dispose behavior are first-class gates; no auto-resume, background service, route picker, Bluetooth/cable/radio scope, duplex or physical-delivery claim is allowed.

The device-acceptance schema and zero-dependency validator now reserve a machine-checkable adapter_observation for future real Android speaker-to-microphone measurements. It requires the v1 adapter contract, API level 26+, exact requested/effective PCM format, granted permission, granted/delayed focus, stop-without-auto-resume and discarded raw PCM. The added JSON fixture is an unexecuted template and contains no device/run/outcome evidence.

Documentation indexes, implementation plan, platform matrix, roadmap, CI bilingual pair checks and public status are synchronized to describe this as a draft design only.

Explicit non-claims

This pull request does not add microphone capture, speaker playback, a Flutter/Android audio plugin, RECORD_AUDIO, audio focus code, an Android target build, Android runtime verification, a device result, speaker-to-microphone delivery, Bluetooth/cable/radio support, encryption or a supported-platform claim.

Validation

The following completed successfully:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• flutter analyze
• flutter test — 10 passing tests and 2 deliberately skipped opt-in native-runtime tests
• flutter build web --release
• flutter build linux --debug
• JSON Schema parse check
• Generic and Android unexecuted device-acceptance template validation
• Extended zero-dependency validator tests
• Bilingual document-pair existence checks
• git diff --check

The web build completes with the existing flutter_rust_bridge wasm dry-run warning. The native bridge remains intentionally unavailable in the web build.