perf(studio): grow composition duration live on extend, no preview remount

Extending a clip past the video end used to force the server-fallback
path that fully remounts the preview iframe (the SDK fast path can't
express the root composition's data-duration, and the runtime bakes+drops
data-duration at load so it can't be patched live). On a large comp that
remount is a visible hitch.

Add a runtime control-bridge action set-root-duration -> clock.setDuration,
so the studio can grow the transport length in place. On an extend the
studio now posts it (and patches the clip's own timing live) instead of
reloading; it only reloads when a GSAP source rewrite actually happened
(the gsap-mutation endpoints now report a mutated flag). Non-animated
extends — the common case — commit as fast as a normal edit.

Verified: bridge dispatch + studio no-reload/post-message paths unit-
tested; core/studio/studio-server typecheck + suites green; the built
runtime artifact carries the handler; E2E confirms the extend no longer
remounts the preview and still persists.

packages/core/src/runtime/init.ts

@@ -1852,6 +1852,7 @@ export function initSandboxRuntimeModular(): void {
   // transport tick. A plain count misses same-count swaps (one sub-comp unloads
   // as another loads), so the signature keys on id+tag in document order.
   let clipTreeSignature = "";
+  let liveRootDurationOverrideSeconds = 0;
   const computeClipTreeSignature = (): string => {
     let sig = "";
     for (const el of document.querySelectorAll("[data-start]")) {
@@ -1904,6 +1905,30 @@ export function initSandboxRuntimeModular(): void {
     scheduleRootStageLayoutDiagnostics();
   };
 
+  const finitePositiveDuration = (value: number): number =>
+    Number.isFinite(value) && value > 0 ? value : 0;
+
+  const growRootDurationLive = (durationSeconds: number) => {
+    const nextDuration = finitePositiveDuration(Number(durationSeconds));
+    if (nextDuration <= 0) return;
+    const rootEl = resolveRootCompositionElement();
+    const rootAttrDuration = finitePositiveDuration(
+      Number.parseFloat(rootEl?.getAttribute("data-duration") ?? ""),
+    );
+    const currentDuration = Math.max(
+      liveRootDurationOverrideSeconds,
+      finitePositiveDuration(clock.getDuration()),
+      rootAttrDuration,
+    );
+    if (nextDuration <= currentDuration) return;
+
+    liveRootDurationOverrideSeconds = nextDuration;
+    rootEl?.setAttribute("data-duration", String(nextDuration));
+    clock.setDuration(nextDuration);
+    postTimeline();
+    postState(true);
+  };
+
   const runAdapters = (method: "discover" | "pause" | "play", timeSeconds = 0) => {
     for (const adapter of state.deterministicAdapters) {
       try {
@@ -2193,6 +2218,7 @@ export function initSandboxRuntimeModular(): void {
       if (state.transportClock) state.transportClock.setRate(state.playbackRate);
       applyWebAudioRate();
     },
+    onSetRootDuration: growRootDurationLive,
     onSetColorGrading: (target, grading) => {
       colorGrading.setGrading(target, grading);
     },