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.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);
   };