feat(phase1): on-device semantic recall — embeddings, vector search, hybrid ranking

Exam-time search over your own lectures, 100% on-device (vectors never leave it):
- EmbeddingEngine (transformers.js feature-extraction via the CDN loader,
  multilingual-e5-small 384-dim, e5 query/passage prefixes); native stub.
- Vector store in StorageRepo (Dexie v3 + native v3 segvecs): upsertVectors,
  brute-force cosine searchVectors (course-scoped), clearVectors, unembeddedIds.
  Cascades: re-embed on segment edit, reassign updates vector courseId, deletes cascade.
- Hybrid search: semantic candidates + lexical rank fused via reciprocal-rank-fusion
  (pure, tested); searchLectures() returns segment hits tagged semantic/lexical/both.
- embeddingStore: build-index/backfill with progress + embed-on-save (fire-and-forget).
- Search screen: query -> segment hits (snippet · course · time) -> tap jumps the
  transcript to that timestamp (seek + scroll-into-view). Per-course stats on Courses.

25 repo tests (incl. cosine ranking + course scoping), 13 search tests, 170 total green.

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

src/app/transcript/[id].tsx

@@ -23,7 +23,12 @@ import { useTranscribe } from '@/stores/transcribeStore';
 
 export default function TranscriptScreen() {
   const theme = useTheme();
-  const { id } = useLocalSearchParams<{ id: string }>();
+  const { id, t } = useLocalSearchParams<{ id: string; t?: string }>();
+  // Deep-link target time (seconds) from a search hit, if any.
+  const jumpTo = useMemo(() => {
+    const n = Number(t);
+    return Number.isFinite(n) && n >= 0 ? n : null;
+  }, [t]);
 
   const [transcript, setTranscript] = useState<Transcript | null | undefined>(undefined);
   const [title, setTitle] = useState('');
@@ -32,6 +37,12 @@ export default function TranscriptScreen() {
   const [saving, setSaving] = useState(false);
   const [currentTime, setCurrentTime] = useState(0);
 
+  const scrollRef = useRef<ScrollView | null>(null);
+  // Y offset of each rendered segment row, captured via onLayout, for scroll-to.
+  const segmentYs = useRef<Record<number, number>>({});
+  // Ensure we only auto-jump once per (id, t) deep-link.
+  const jumpedRef = useRef(false);
+
   // Prefer the just-transcribed in-session audio; otherwise load the persisted
   // source media so playback/scrub works after a reload (ROADMAP Phase 0).
   const sessionAudioUrl = useTranscribe((s) => (s.lastTranscriptId === id ? s.audioUrl : undefined));
@@ -90,13 +101,48 @@ export default function TranscriptScreen() {
     [segments, currentTime],
   );
 
-  const seek = (t: number) => {
+  const seek = (time: number) => {
     const el = audioRef.current;
     if (!el) return;
-    el.currentTime = t;
+    el.currentTime = time;
     void el.play();
   };
 
+  // Reset the one-shot jump guard whenever the deep-link target changes.
+  useEffect(() => {
+    jumpedRef.current = false;
+  }, [id, jumpTo]);
+
+  // Deep-link jump: once the transcript is loaded and we have a finite `t`,
+  // seek the audio (if present), highlight the matching segment, and scroll it
+  // into view. Works even without audio (scroll/highlight by time only).
+  useEffect(() => {
+    if (jumpTo === null || jumpedRef.current) return;
+    if (!transcript || segments.length === 0) return;
+
+    const target = segments.findIndex((s) => jumpTo >= s.start && jumpTo < s.end);
+    const idx = target >= 0 ? target : nearestIndex(segments, jumpTo);
+    if (idx < 0) return;
+
+    jumpedRef.current = true;
+    // Drive the highlight by time even if audio isn't available.
+    setCurrentTime(jumpTo);
+    if (audioRef.current) seek(jumpTo);
+
+    // Scroll once the row's Y offset has been measured (next frames).
+    const scrollToIdx = () => {
+      const y = segmentYs.current[idx];
+      if (y === undefined) return false;
+      scrollRef.current?.scrollTo({ y: Math.max(0, y - Spacing.four), animated: true });
+      return true;
+    };
+    if (!scrollToIdx()) {
+      // Layout may not be measured yet; retry on the next ticks.
+      const timers = [requestAnimationFrame(() => { if (!scrollToIdx()) setTimeout(scrollToIdx, 120); })];
+      return () => timers.forEach(cancelAnimationFrame);
+    }
+  }, [jumpTo, transcript, segments, audioUrl]);
+
   const editSegment = (i: number, text: string) => {
     setSegments((prev) => prev.map((s, idx) => (idx === i ? { ...s, text } : s)));
     setDirty(true);
@@ -130,11 +176,11 @@ export default function TranscriptScreen() {
   return (
     <ThemedView style={styles.fill}>
       <Stack.Screen options={{ title: title || 'Transcript' }} />
-      <ScrollView contentContainerStyle={styles.content}>
+      <ScrollView ref={scrollRef} contentContainerStyle={styles.content}>
         <TextInput
           value={title}
-          onChangeText={(t) => {
-            setTitle(t);
+          onChangeText={(next) => {
+            setTitle(next);
             setDirty(true);
           }}
           style={[styles.title, { color: theme.text }]}
@@ -160,7 +206,12 @@ export default function TranscriptScreen() {
         </View>
 
         {segments.map((s, i) => (
-          <View key={i} style={[styles.segRow, i === activeIndex && { backgroundColor: theme.backgroundSelected }]}>
+          <View
+            key={i}
+            onLayout={(e) => {
+              segmentYs.current[i] = e.nativeEvent.layout.y;
+            }}
+            style={[styles.segRow, i === activeIndex && { backgroundColor: theme.backgroundSelected }]}>
             <Pressable onPress={() => seek(s.start)} hitSlop={6}>
               <ThemedText type="code" themeColor="textSecondary" style={styles.ts}>
                 {formatClock(s.start)}
@@ -168,7 +219,7 @@ export default function TranscriptScreen() {
             </Pressable>
             <TextInput
               value={s.text}
-              onChangeText={(t) => editSegment(i, t)}
+              onChangeText={(next) => editSegment(i, next)}
               multiline
               style={[styles.segText, { color: theme.text }]}
             />
@@ -192,6 +243,24 @@ export default function TranscriptScreen() {
   );
 }
 
+/** Index of the segment whose start is closest to `time` (fallback for jumps
+ *  that don't land inside any segment's [start, end) window). */
+function nearestIndex(segments: Segment[], time: number): number {
+  if (segments.length === 0) return -1;
+  let best = 0;
+  let bestDist = Infinity;
+  for (let i = 0; i < segments.length; i++) {
+    const seg = segments[i];
+    if (!seg) continue;
+    const dist = Math.abs(seg.start - time);
+    if (dist < bestDist) {
+      bestDist = dist;
+      best = i;
+    }
+  }
+  return best;
+}
+
 function Centered({ children }: { children: React.ReactNode }) {
   return <ThemedView style={[styles.fill, styles.centered]}>{children}</ThemedView>;
 }