fix(A.3 #24): clamp LocalAgreement slices against latest.len() to prevent panic on shrinking passes

Review feedback (CRITICAL): LocalAgreement::push could panic with an
index OOB when a later pass arrived shorter than committed_count.
Concrete case: commit [a, b], next pass arrives [a] — lcp_len=1,
new_committed=max(1, 2)=2, then latest[2..] panicked because
latest.len()==1.

A Whisper re-transcription of an overlapping window can legitimately
collapse repeated segments, or the user can stop mid-utterance after
some tokens were already committed, both of which produce this
shape. The committed_count invariant still holds (non-shrinkage) —
it is the slicing that was unsafe.

Clamps every latest[..] slice against latest.len() before indexing.
committed_count stays at new_committed even when the pass is shorter:
non-shrinkage is relative to what we have already emitted, not to
the current pass length. newly_committed and tentative both return
empty when the shorter pass has nothing past the committed prefix.

Adds two regression tests:
- shorter_pass_after_commit_does_not_panic (commit 2, push 1)
- empty_pass_after_commit_does_not_panic (commit 1, push empty)

crates/transcription/src/streaming/commit_policy.rs

@@ -124,11 +124,21 @@ impl LocalAgreement {
         // The agreed prefix can only grow — never shrink below what we
         // already committed. ufal's invariant: once committed, stay
         // committed.
-        let new_lcp_len = lcp_len.max(self.committed_count);
+        let new_committed = lcp_len.max(self.committed_count);
 
         let latest = self.history.back().expect("history is non-empty here");
-        let newly_committed = if new_lcp_len > self.committed_count {
-            latest[self.committed_count..new_lcp_len].to_vec()
+        // Clamp every slice against `latest.len()` — a later pass can
+        // legitimately arrive shorter than `committed_count` (Whisper
+        // re-transcribing an overlapping window with fewer segments,
+        // or user stopping mid-word while the committer holds a longer
+        // history). Without the clamp, `latest[committed_count..]`
+        // panics with an index OOB.
+        let old_committed = self.committed_count;
+        let latest_len = latest.len();
+        let emit_start = old_committed.min(latest_len);
+        let emit_end = new_committed.min(latest_len);
+        let newly_committed = if emit_end > emit_start {
+            latest[emit_start..emit_end].to_vec()
         } else {
             Vec::new()
         };
@@ -136,9 +146,14 @@ impl LocalAgreement {
         if let Some(last) = newly_committed.last() {
             self.last_committed_end_secs = last.end_secs;
         }
-        self.committed_count = new_lcp_len;
+        // `committed_count` stays at `new_committed` even when the
+        // latest pass is shorter — the non-shrinkage invariant holds
+        // relative to what we've already emitted, not to the current
+        // pass length.
+        self.committed_count = new_committed;
 
-        let tentative = latest[new_lcp_len..].to_vec();
+        let tentative_start = new_committed.min(latest_len);
+        let tentative = latest[tentative_start..].to_vec();
 
         CommitDecision {
             newly_committed,
@@ -303,6 +318,37 @@ mod tests {
         assert!(decision.newly_committed.is_empty());
     }
 
+    #[test]
+    fn shorter_pass_after_commit_does_not_panic() {
+        // Regression: committed_count = 2, then a pass arrives with
+        // only 1 token (Whisper re-transcribing an overlapping window
+        // that collapses repeated segments, or user stopping mid-
+        // utterance). `latest[committed_count..]` would index OOB.
+        let mut la = LocalAgreement::new(2);
+        let _ = la.push(vec![tok("a", 0.0, 0.1), tok("b", 0.1, 0.2)]);
+        let _ = la.push(vec![tok("a", 0.0, 0.1), tok("b", 0.1, 0.2)]);
+        assert_eq!(la.committed_count, 2);
+
+        let decision = la.push(vec![tok("a", 0.0, 0.1)]);
+        // committed_count stays at 2 (non-shrinkage invariant).
+        assert_eq!(la.committed_count, 2);
+        // No new commit, no tentative (nothing past position 2 in the
+        // shorter pass).
+        assert!(decision.newly_committed.is_empty());
+        assert!(decision.tentative.is_empty());
+    }
+
+    #[test]
+    fn empty_pass_after_commit_does_not_panic() {
+        let mut la = LocalAgreement::new(2);
+        let _ = la.push(vec![tok("a", 0.0, 0.1)]);
+        let _ = la.push(vec![tok("a", 0.0, 0.1)]);
+        let decision = la.push(vec![]);
+        assert_eq!(la.committed_count, 1);
+        assert!(decision.newly_committed.is_empty());
+        assert!(decision.tentative.is_empty());
+    }
+
     #[test]
     fn flush_emits_remaining_tentative() {
         let mut la = LocalAgreement::new(2);