feat(studio): stage 7 step 3c — sdk cutover for inline-style ops (#1522)

Introduces sdkCutoverPersist(): when STUDIO_SDK_CUTOVER_ENABLED is set,
inline-style PatchOps are routed through the SDK session's in-memory document
model instead of the server patch-element API. The SDK serialize() result is
written back through the same writeProjectFile + editHistory.recordEdit path,
so the on-disk output is identical to the legacy route.

- packages/studio/src/utils/sdkCutover.ts (new): sdkCutoverPersist() +
  shouldUseSdkCutover() guard; domEditSaveTimestampRef.current is stamped on
  each write to suppress the echo file-change reload.
- packages/studio/src/components/editor/manualEditingAvailability.ts: adds
  STUDIO_SDK_CUTOVER_ENABLED flag (default false); changes
  STUDIO_SDK_SHADOW_ENABLED default to false now that cutover is available.
- packages/studio/src/hooks/useSdkSession.ts: adds optional
  domEditSaveTimestampRef param; self-write suppress window (SELF_WRITE_SUPPRESS_MS)
  gates file-change reloads so SDK writes don't echo back as external edits.
- packages/studio/src/App.tsx: passes domEditSaveTimestampRef to useSdkSession
  so the suppress window can gate reloads triggered by SDK cutover writes.
- Test coverage: sdkCutover.test.ts (new, 141 lines) + useDomEditSession.test.ts
  (new, 50 lines) — guard function + happy-path assertions.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/studio/src/App.tsx

@@ -155,6 +155,7 @@ export function StudioApp() {
     domEditSaveTimestampRef,
     setRefreshKey,
   });
+  const sdkSession = useSdkSession(projectId, activeCompPath, domEditSaveTimestampRef);
   useEffect(() => {
     if (activeCompPathHydrated) return;
     if (!fileManager.fileTreeLoaded) return;
@@ -177,7 +178,6 @@ export function StudioApp() {
     reloadPreview: () => setRefreshKey((k) => k + 1),
     pendingTimelineEditPathRef,
   });
-  const sdkSession = useSdkSession(projectId, activeCompPath ?? "index.html");
   const timelineEditing = useTimelineEditing({
     projectId,
     activeCompPath,