feat: add constrained live-audio adapter contract

[matveyb9]

Summary

This change introduces the first constrained live-audio adapter contract without enabling a physical audio route.

It adds the canonical English RFC and Russian translation, a typed Flutter LiveAudioAdapter boundary, and an UnavailableLiveAudioAdapter default. The default reports 48000 Hz, mono, signed PCM16 little-endian as the required future stream format while refusing playback and capture truthfully. No audio plugin is initialized, no permission is requested, and no platform/device route is claimed.

The workbench now accepts the contract as a dependency and shows the adapter's unavailable reason when a user selects the future live route. New tests cover the PCM format invariant, unavailable adapter behavior, live-route selection, the existing send flow, and the existing import flow.

Documentation now includes lifecycle, capability, shutdown, error and device-acceptance requirements; research sources for permission/session/focus constraints; bilingual indexes and roadmap status; public status wording; and CI checks for the added English/Russian file pairs.

Scope boundary

This is an unavailable-first scaffold only. It does not add a recording/playback plugin, microphone permission, audio session configuration, hardware route, Bluetooth route, radio interface, physical-route result, supported-device claim, encryption or automatic recovery behavior.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test
• 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/test-validator.mjs
• git diff --check

The web build completes with the existing flutter_rust_bridge wasm dry-run warning; the bridge is intentionally unavailable on web and the warning does not fail the build.