perf(ext/content): debounce MutationObserver scan() to 200ms (Plan C Phase 5)

DEV-C P2: SPA churn was re-running the full scan many times per second.
Trailing-edge debounce coalesces bursts so scan() runs at most once per
quiet 200ms window.

No test harness exists for content/detector.ts on this branch; relies
on manual verification on a real SPA page.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

extension/src/content/detector.ts

@@ -93,9 +93,21 @@ setupFillListener();
 scan();
 
 // Watch for DOM changes (SPA navigation, dynamically loaded forms).
-const observer = new MutationObserver(() => {
+// Plan C Phase 5: SPA churn fires the MutationObserver many times per
+// second. Trailing-edge debounce coalesces bursts so we run the full
+// scan() at most once per quiet 200ms window.
+const SCAN_DEBOUNCE_MS = 200;
+let scanTimer: ReturnType<typeof setTimeout> | undefined;
+
+function scheduleScan(): void {
+  if (scanTimer !== undefined) clearTimeout(scanTimer);
+  scanTimer = setTimeout(() => {
+    scanTimer = undefined;
     scan();
-});
+  }, SCAN_DEBOUNCE_MS);
+}
+
+const observer = new MutationObserver(scheduleScan);
 
 observer.observe(document.body, {
   childList: true,