feat: 5-frame button assets + timeline/sidebar polish

Button assets
- Re-introduce button-asset type with 4 content states (normal/hover/
  pressed/disabled) stored as keyframes at BUTTON_STATE_TIMES, plus a
  separate hit-area editor mode.
- useButtonRuntime composable for per-instance state/disabled flags on
  the main stage.
- Sidebar library lists button assets with live 80×80 thumbnails;
  renderButtonThumbnail now accepts an images map so image-kind layers
  inside a button render instead of producing a fully-transparent PNG.
- Compact (no-thumbnail) mode aligns button rows to the library-row
  height.

Scope footer
- Button-edit mode now uses the same sidebar scope-footer pattern as
  clip-edit (back button to "Main timeline" + active scope chip);
  removed the inline exit button from the timeline scrub-bar.

Timeline
- Track-name column default bumped to 260px to match the assets sidebar
  above it (storage key bumped to :v2 so stale values reset).
- scrub-bar z-index raised above sticky track-name columns so rows
  scroll under it instead of over.
- scrub-handle flips z-index based on horizontal scroll: above
  name-spacer when the playhead is in view (no half-clip at t=0),
  below when the playhead has scrolled off the left edge (so the
  toggle row cleanly covers it).
- name-spacer extends ±3/1px to fully cover the obscured handle.
- scrub-handle head pokes 3px above the ruler with rounded top corners
  via clip-path path().

Track context menu
- Added missing i18n keys (renameScriptTrack / deleteScriptTrack /
  renameScriptTrackDialog.{title,confirm}).
- Rename items show a visible pencil icon; empty .check slot removed
  so items line up flush-left; toggle items place their active ✓ on
  the right (kf-menu-trailing-check).

Dev HUD
- Test progress bar flips to --danger as soon as the first failure is
  recorded, instead of only at the end of the run.

Regression tests
- renderButtonThumbnail: null on empty button, renders shape layers,
  accepts custom images map.
- i18n: renameScriptTrack / deleteScriptTrack / dialog keys resolve.
- scrub-handle-obscured logic (visible at t=0, hidden when playhead is
  scrolled off-screen, still visible when playhead is far to the
  right of the scroll offset).
- Updated button state-scrubber expectations to match
  BUTTON_STATE_TIMES (normal=1, hover=2, pressed=3, disabled=4) and
  that hit-mode parks at normal=1 rather than 0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/interpolate.ts

@@ -128,6 +128,39 @@ function sampleKeyframeSet(
   return { ...base }
 }
 
+/**
+ * Union range of all keyframe times on a layer (main + position + rotation
+ * tracks). A layer is considered "alive" only within this range.
+ *
+ * Returns null (→ always visible) when the layer has zero or one total
+ * keyframes — a single keyframe is a static layer with no animation range.
+ */
+export function layerActiveRange(
+  layer: Layer,
+): { min: number; max: number } | null {
+  const main = layer.keyframes || []
+  const pos = layer.positionKeyframes || []
+  const rot = layer.rotationKeyframes || []
+  const total = main.length + pos.length + rot.length
+  if (total <= 1) return null
+  let min = Infinity
+  let max = -Infinity
+  for (const arr of [main, pos, rot]) {
+    for (const k of arr) {
+      if (k.time < min) min = k.time
+      if (k.time > max) max = k.time
+    }
+  }
+  return { min, max }
+}
+
+export function isLayerActiveAt(layer: Layer, time: number): boolean {
+  const range = layerActiveRange(layer)
+  if (!range) return true
+  const EPSILON = 1e-6
+  return time >= range.min - EPSILON && time <= range.max + EPSILON
+}
+
 export function sampleLayer(layer: Layer, time: number): AnimatableProps {
   const main = sampleKeyframeSet(layer.keyframes, layer.base, time)
   if (!layer.separated) return main