feat: add bounded file WAV workflow

[matveyb9]

Summary

This implementation slice follows the new test-gated execution plan and adds the first bounded local file-object WAV workflow. It does not add a physical audio route.

Rust now exposes an ADLP File constructor with required file-name and MIME metadata, a deterministic binary-payload round-trip test, a committed golden WAV fixture, and reproducible adlp-cli encode-file / encode-acoustic1-file commands. The thin Flutter Rust facade accepts or decodes files with an initial 8 KiB payload limit, while preserving Rust as the sole authority for ADLP framing, CRC-32C and WAV codec behavior.

The Flutter workbench now has explicit transfer-task states, a text/file object selector, local payload selection, verified text/file receipts, and local save for only Rust-verified received file bytes. The normal widget suite remains portable through fakes. An opt-in Linux smoke test loads the freshly built native shared library and exercises the real generated Flutter-to-Rust file bridge.

The documentation adds a bilingual implementation plan, CLI file walkthrough, bridge contract, native smoke command, roadmap/status updates and the corresponding English/Russian pair check.

Scope boundary

This is an experimental local WAV reference workflow. It does not provide microphone capture, speaker playback, cables, Bluetooth, radio, background transfer, persistence/history, a file-size policy beyond the bridge's initial 8 KiB limit, authentication, encryption, device support or a live-audio 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
• AUDIOMODEM_RUN_NATIVE_BRIDGE_SMOKE=1 LD_LIBRARY_PATH="$PWD/build/linux/x64/debug/bundle/lib" flutter test test/native_bridge_smoke_test.dart
• Device-acceptance schema/validator fixture 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.