Bug-hunt sweep: fix 13 verified issues (correctness + perf)

From a multi-agent bug-hunt + adversarial verification pass (0 critical/
high; 15 mediums). Fixed 13; 2 deferred as bigger refactors.

Correctness:
- srs: "Hard" no longer overshoots "Good" for reviewed cards (reps>=2). It
  compounded ease AND x1.2; now grows x1.2 off the previous interval only.
  + regression test.
- enrich/dates: stop reading the modal verb "may" as the month May (phantom
  calendar events). "may" needs an ordinal/year/date-preposition now.
- enrich/bib: disambiguate colliding BibTeX keys (smith2020, smith2020a, …)
  — duplicates corrupted reference-manager imports.
- learn/glossary: junk-term guard used && (dead); now || so all-stopword
  terms like "there" are actually skipped.
- transcription/engineImpl.web: don't collapse Whisper's null end-timestamp
  to a zero-length [t,t] segment; estimate from the next chunk or window
  duration (fixes citation/seam anchors).
- transcription/pipeline: re-check the abort signal AFTER each chunk so
  Cancel works on single-chunk audio (it previously still saved).
- db/repo.native: use withExclusiveTransactionAsync for reassign /
  deleteCourse / upsertVectors / createFlashcards (withTransactionAsync is
  not isolated on a shared connection → interleaved/half-applied writes).
- db/repo.native: don't cache a rejected open/migrate promise — a transient
  first-open failure no longer bricks storage for the whole session.
- stores/transcriptsStore: sequence-guard refresh() so overlapping
  focus/typing/filter refreshes can't resolve out of order and show stale
  results.
- audio/wav: decode WAVE_FORMAT_EXTENSIBLE (0xFFFE) via its SubFormat GUID
  + add 24-bit PCM — common ffmpeg/Windows WAVs no longer hard-fail native
  import. + tests.

Performance / footprint:
- audio/decode.native: decode straight to 16kHz (decodeAudioData sampleRate
  hint) so the JS side never holds a full-rate buffer or runs the resample
  loop — big memory/OOM win on long lectures.
- models/catalog display: Settings model sizes are now backend-aware (the
  no-GPU WASM path pulls ~2x fp32 weights; was advertising ~half).

Feature gap:
- download: native exports were silent no-ops. New download.native.ts writes
  to cache + opens the share sheet (expo-sharing); transcript/ICS/Anki-CSV/
  BibTeX/RIS exports now work on device.

Deferred (bigger): lexical-search recall ceiling (needs a full-corpus rank
path on both repos); triple in-memory copy of the encoded file during
transcribe (media is keyed by a not-yet-existing transcript id).

Validated: tsc clean, 282 tests pass, web export clean (native deps not
bundled), arm64 APK compiles with expo-sharing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/transcription/engineImpl.web.ts

@@ -156,13 +156,26 @@ export const engine: TranscriptionEngine = {
 
     const out = await asr(audio.samples, genOpts);
 
+    // Whisper sometimes omits the END timestamp of the last utterance in a
+    // window (transformers.js yields timestamp[1] === null). Don't collapse that
+    // to a zero-length [t, t] segment (it breaks stitch seams + citation spans):
+    // estimate the end from the next chunk's start, else the window's duration.
+    const chunks = out.chunks ?? [];
+    const chunkDurationSec = audio.samples.length / audio.sampleRate;
     const segments: Segment[] = [];
-    for (const c of out.chunks ?? []) {
-      const [start, end] = c.timestamp;
+    for (let i = 0; i < chunks.length; i++) {
+      const c = chunks[i]!;
+      const [start, rawEnd] = c.timestamp;
       if (start == null) continue;
       const text = c.text.trim();
       if (text.length === 0) continue;
-      segments.push({ start, end: end ?? start, text });
+      let end = rawEnd;
+      if (end == null) {
+        const nextStart = chunks[i + 1]?.timestamp[0];
+        end = nextStart != null && nextStart > start ? nextStart : chunkDurationSec;
+      }
+      if (end < start) end = chunkDurationSec;
+      segments.push({ start, end, text });
     }
     return segments;
   },