feat(A.3 #24): LocalAgreement-n commit policy

New streaming::commit_policy module implementing ufal's
LocalAgreement-n pattern: tokens emitted by the streaming ASR
pipeline stay tentative until N consecutive passes produce the same
prefix, at which point the agreed prefix commits.

Types:
- Token: text + absolute start/end seconds. PartialEq is text-only so
  identical words from overlapping Whisper windows compare equal even
  when their timestamps drift.
- CommitDecision { newly_committed, tentative }: the partition fed
  back to the live-session worker after each pass.
- CommitPolicy enum with LocalAgreement { n }. Default is n=2 (ufal).
- LocalAgreement: stateful committer with push/flush/reset and a
  last_committed_end_secs accessor. Brief item #25 uses the latter
  to compute the sample-index drain target for aggressive buffer trim.

Invariants exercised by tests:
- first pass is all tentative (need 2 passes to commit under n=2)
- two matching passes commit their common prefix
- divergent second pass commits nothing
- extending agreement commits only the newly-agreed tokens
- tentative tail tracks latest pass only (no stale guesses)
- committed prefix never shrinks, even if later passes contradict
- flush emits any tentative-but-not-committed tail at session end
- flush on empty history is a no-op
- reset clears all commit state
- n=3 requires three matching passes before anything commits
- CommitPolicy::default() is LocalAgreement { n: 2 }

Integration into src-tauri/src/commands/live.rs deferred — the
tentative/committed split needs the B-side 'tentative: bool' field on
LiveResultMessage.segments (workstream-B #24 UI contract) and
validation against real streaming captures before going live.

crates/transcription/src/streaming/mod.rs

@@ -6,8 +6,10 @@
 //! threshold tuning can be validated against real microphone captures
 //! rather than synthetic fixtures (brief items #21, #24, #25).
 
+pub mod commit_policy;
 pub mod rms_vad;
 
+pub use commit_policy::{CommitDecision, CommitPolicy, LocalAgreement, Token};
 pub use rms_vad::RmsVadChunker;
 
 /// A span of audio the VAD considers worth transcribing. `start_sample`