Smart word timing redistribution on transcript edits

When editing a segment, word timing is now intelligently redistributed:
- Spelling fixes (same word count): each word keeps its original timing
- Word splits (e.g. "gonna" → "going to"): original word's time range
  is divided proportionally across the new words
- Inserted words: timing interpolated from neighboring words
- Deleted words: remaining words keep their timing, gaps collapse

This preserves click-to-seek accuracy for common edits like fixing
misheard words or splitting concatenated words.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/components/TranscriptEditor.svelte

@@ -63,25 +63,7 @@
       segments.update(segs => segs.map(s => {
         if (s.id !== segmentId) return s;
         const newWordTexts = trimmed.split(/\s+/);
-        let newWords;
-        if (newWordTexts.length === s.words.length) {
-          // Same word count (e.g. spelling fix) — preserve each word's timing
-          newWords = s.words.map((w, widx) => ({
-            ...w,
-            word: newWordTexts[widx],
-          }));
-        } else {
-          // Word count changed — fall back to segment-level timing
-          newWords = newWordTexts.map((word, widx) => ({
-            id: `${s.id}-word-${widx}`,
-            segment_id: s.id,
-            word,
-            start_ms: s.start_ms,
-            end_ms: s.end_ms,
-            confidence: 1.0,
-            word_index: widx,
-          }));
-        }
+        const newWords = redistributeWords(s, newWordTexts);
         return {
           ...s,
           text: trimmed,
@@ -96,6 +78,106 @@
     editingSegmentId = null;
   }
 
+  /**
+   * Redistribute word timing after an edit.
+   *
+   * Uses a diff-like alignment between old and new word lists:
+   * - Unchanged words keep their original timing
+   * - Spelling fixes (same position, same count) keep timing
+   * - Split words (1 old → N new) divide the original time range proportionally
+   * - Inserted words with no match get interpolated timing
+   */
+  function redistributeWords(segment: Segment, newWordTexts: string[]): Word[] {
+    const oldWords = segment.words;
+
+    // Same word count — preserve per-word timing (spelling fixes)
+    if (newWordTexts.length === oldWords.length) {
+      return oldWords.map((w, i) => ({ ...w, word: newWordTexts[i] }));
+    }
+
+    // Align old words to new words using a simple greedy match.
+    // Build a mapping: for each old word, which new words does it cover?
+    const oldTexts = oldWords.map(w => w.word.toLowerCase());
+    const newTexts = newWordTexts.map(w => w.toLowerCase());
+
+    // Walk both lists, greedily matching old words to new words
+    const result: Word[] = [];
+    let oldIdx = 0;
+    let newIdx = 0;
+
+    while (newIdx < newTexts.length) {
+      if (oldIdx < oldTexts.length && oldTexts[oldIdx] === newTexts[newIdx]) {
+        // Exact match — keep original timing
+        result.push({ ...oldWords[oldIdx], word: newWordTexts[newIdx], word_index: newIdx });
+        oldIdx++;
+        newIdx++;
+      } else if (oldIdx < oldTexts.length) {
+        // Check if old word was split into multiple new words.
+        // E.g., "gonna" → "going to": see if concatenating upcoming new words
+        // matches the old word (or close enough — just check if old word's chars
+        // are consumed by the next few new words).
+        let splitCount = 0;
+        let combined = '';
+        for (let k = newIdx; k < newTexts.length && k - newIdx < 5; k++) {
+          combined += (k > newIdx ? '' : '') + newTexts[k];
+          if (combined.length >= oldTexts[oldIdx].length) {
+            splitCount = k - newIdx + 1;
+            break;
+          }
+        }
+
+        if (splitCount > 1) {
+          // Split: distribute the old word's time range proportionally
+          const ow = oldWords[oldIdx];
+          const totalDuration = ow.end_ms - ow.start_ms;
+          for (let k = 0; k < splitCount; k++) {
+            const fraction = 1 / splitCount;
+            result.push({
+              id: `${segment.id}-word-${newIdx + k}`,
+              segment_id: segment.id,
+              word: newWordTexts[newIdx + k],
+              start_ms: Math.round(ow.start_ms + totalDuration * fraction * k),
+              end_ms: Math.round(ow.start_ms + totalDuration * fraction * (k + 1)),
+              confidence: ow.confidence,
+              word_index: newIdx + k,
+            });
+          }
+          oldIdx++;
+          newIdx += splitCount;
+        } else {
+          // No match found — interpolate timing from neighbors
+          const prevEnd = result.length > 0 ? result[result.length - 1].end_ms : segment.start_ms;
+          const nextStart = oldIdx < oldWords.length ? oldWords[oldIdx].start_ms : segment.end_ms;
+          result.push({
+            id: `${segment.id}-word-${newIdx}`,
+            segment_id: segment.id,
+            word: newWordTexts[newIdx],
+            start_ms: prevEnd,
+            end_ms: nextStart,
+            confidence: 1.0,
+            word_index: newIdx,
+          });
+          newIdx++;
+        }
+      } else {
+        // No more old words — use end of segment
+        const prevEnd = result.length > 0 ? result[result.length - 1].end_ms : segment.start_ms;
+        result.push({
+          id: `${segment.id}-word-${newIdx}`,
+          segment_id: segment.id,
+          word: newWordTexts[newIdx],
+          start_ms: prevEnd,
+          end_ms: segment.end_ms,
+          confidence: 1.0,
+          word_index: newIdx,
+        });
+        newIdx++;
+      }
+    }
+
+    return result;
+  }
+
   function handleEditKeydown(e: KeyboardEvent, segmentId: string) {
     if (e.key === 'Escape') {
       editingSegmentId = null;