fix(studio): fix array-form keyframe writes, diamond click-deselect, and nested video sync

- fs.watch's async 'error' event had no listener, crashing the preview
  server on EMFILE (exhausted OS watch handles)
- moveKeyframeInScript/resizeKeyframedTweenInScript/removeAllKeyframesFromScript
  required object-form keyframes: {"0%": {...}}, silently no-opping on
  array-form keyframes: [{...}, {...}]
- a keyframe diamond click's auto-synthesized native click event bubbled
  to the ancestor clip's onClick, which toggles selection off when the
  clip is already selected (the state every diamond click happens in)
- the clip's trim-resize handles (z-index 4) visually and functionally
  covered any keyframe diamond within their 14px edge strip
- synthesizeFlatTweenKeyframes didn't recognize a collapsed
  duration:0 + immediateRender static hold (what remove-all-keyframes
  produces) as non-animated, so it kept showing a phantom diamond after
  Delete All Keyframes
- resolveMediaStartSeconds's fast path for elements with their own
  data-start discarded the host composition's inherited start offset,
  so a video nested inside a sub-composition played from the root
  timeline's time instead of holding until its parent scene began

Fixes #1838

packages/core/src/runtime/init.test.ts

@@ -550,6 +550,56 @@ describe("initSandboxRuntimeModular", () => {
     expect(video.currentTime).toBe(9);
   });
 
+  // Regression (#1838): a video authoring its OWN data-start (the normal case
+  // for a timed clip the studio positions on a track) took a fast literal-
+  // value path that skipped adding the host composition's start offset —
+  // unlike the no-own-data-start case above, which already went through
+  // resolveStartForElement and got the offset for free. The video played
+  // from the ROOT timeline's time instead of holding until its parent scene
+  // began, desyncing from the correctly-offset GSAP overlay in the same scene.
+  it("offsets a nested video's own data-start by its host composition's start", () => {
+    const root = document.createElement("div");
+    root.setAttribute("data-composition-id", "main");
+    root.setAttribute("data-root", "true");
+    root.setAttribute("data-width", "1920");
+    root.setAttribute("data-height", "1080");
+    document.body.appendChild(root);
+
+    const child = document.createElement("div");
+    child.setAttribute("data-composition-id", "scene-2");
+    child.setAttribute("data-start", "20");
+    child.setAttribute("data-duration", "16");
+    root.appendChild(child);
+
+    const video = document.createElement("video");
+    // Authored relative to scene-2's own local timeline, not the root's.
+    video.setAttribute("data-start", "0");
+    video.setAttribute("data-duration", "16");
+    child.appendChild(video);
+    Object.defineProperty(video, "duration", { value: 20, writable: true, configurable: true });
+    Object.defineProperty(video, "paused", { value: true, writable: true, configurable: true });
+    Object.defineProperty(video, "readyState", { value: 4, writable: true, configurable: true });
+    Object.defineProperty(video, "currentTime", { value: 0, writable: true, configurable: true });
+    video.load = () => {};
+    video.play = () => Promise.resolve();
+
+    window.__timelines = {
+      main: createMockTimeline(40),
+      "scene-2": createMockTimeline(16),
+    };
+
+    initSandboxRuntimeModular();
+
+    const player = window.__player;
+    expect(player).toBeDefined();
+
+    // Root t=25 is 5s into scene-2 (which starts at root t=20) — the video
+    // must be 5s into its own local playback, not 25s (root time).
+    player?.seek(25);
+
+    expect(video.currentTime).toBe(5);
+  });
+
   it("updates visibility for timed elements inside nested compositions", () => {
     const root = document.createElement("div");
     root.setAttribute("data-composition-id", "main");

packages/core/src/runtime/init.ts

@@ -496,7 +496,16 @@ export function initSandboxRuntimeModular(): void {
 
   const resolveMediaStartSeconds = (element: Element, fallback = 0): number => {
     if (!element.hasAttribute("data-hf-auto-start") && element.hasAttribute("data-start")) {
-      return Math.max(0, Number(element.getAttribute("data-start") ?? 0) || 0);
+      // `data-start` is authored relative to the media element's OWN sub-
+      // composition, not the root timeline — `fallback` carries the host
+      // composition's resolved absolute start (see syncMediaForCurrentState's
+      // inheritedStart), so it must be added, not discarded. Skipping it made
+      // a nested video play from root t=0 instead of holding until its
+      // parent scene began (issue #1838) — resolveStartForElement's own
+      // absolute-expression branch already adds this same host offset, this
+      // fast literal-value path just didn't.
+      const own = Math.max(0, Number(element.getAttribute("data-start") ?? 0) || 0);
+      return own + fallback;
     }
     return resolveStartForElement(element, fallback);
   };