feat(dart): add idiomatic voice runtime interfaces

[Pushkraj-Space]

Summary

Issue #34 needs one idiomatic Dart runtime surface that wearables, phone microphones, headsets, fixtures, and future connectors can implement without coupling host code to Flutter, hardware, providers, storage, or generated protobuf classes.

This change:

• replaces the Dart SDK's opaque SessionControl body and audio-format map with typed murmur.v1 models
• preserves the presence of known default-valued fields, including open: false, frameDurationMs: 0, and empty command objects
• adds pure-Dart VoiceConnector and VoiceSession interfaces plus SessionState and typed VoiceError
• defines terminal session, cancellation, error ordering, stream buffering, and shared cleanup behavior in the public API documentation
• adds a deterministic test-only connector/session fake covering discovery, idle connection, capture, cancellation races, late acquisition cleanup, terminal device errors, paused and unlistened streams, and bounded frame dropping
• updates the Dart SDK documentation and clarifies the checked-in fixture presence rule in the conformance documentation

Wire compatibility

AudioEncoding, AudioFormat, and CaptureMode map directly to the existing murmur.v1 schema. StartSession, StopSession, SetInputGate, and FinalizeSession model the four existing control oneof variants, with nullable fields representing absence.

AudioFrame.payloadBase64 remains verbatim, while its format is now validated and typed. Session-scoped sequence and producing-host monotonic timestamp semantics are documented on the model.

This PR does not change protobuf definitions, shared fixtures, events.dart, source.dart, or other language SDKs.

Runtime lifecycle

• connect returns an idle session and capture begins only through start
• stopped and error sessions are terminal, with getters remaining readable
• concurrent and repeated stop/close calls share one cleanup future
• a winning stop/close cancels a pending start with VoiceError(code: "cancelled")
• late native acquisition is released without state or frame resurrection
• device failures publish the error cause before the error-state transition
• cleanup closes streams without awaiting consumer-dependent drainage
• the test fake buffers at most two undelivered frames and drops the oldest

Validation

• make check-dart — format, analyzer, shared conformance tests, and 19 total Dart tests pass
• make check-flutter — analyzer and both Omi application tests pass
• git diff --check

Closes #34

[harshsaver]

Thanks @Pushkraj-Space. The typed models map 1:1 to session.proto, the old untyped API is gone with no shim, and presence-preserving nullables are justified by the "open": false fixture. make check-dart passes locally (19 tests).

Optional nits (fine as follow-ups):

• lib/src/runtime.dart: close() is documented as identical to stop(). Unless a host needs both names, one method is simpler.
• lib/src/runtime.dart: the in-process SessionState shares its name with the wire enum but has no warmMuted. A one-line doc note saying warm-muted belongs to the coordinator (per docs/voice-runtime.md) would prevent confusion.
• conformance/README.md:36-37: "Accepted fixtures are canonical: default-valued fields are omitted" now sits right next to a rule about preserving explicit defaults, and session-control.jsonl:3 has one. Please reconcile the two sentences.
• test/runtime_test.dart:471-773: feat(dart): add provider-neutral voice capture coordinator #51 adds a second fake in test/support/. Consider moving this one there so both share it.

Merging; please rebase #51 afterwards.