0.8.5: multi-select tracks, group + convert-to-clip

Multi-select
- New `selectedLayerIds: string[]` on the scene store — ordered by
  click, anchor = last entry, always equal to `selectedLayerId`.
- `toggleLayerSelection(id)` shift-click behaviour: adds or removes,
  anchor follows the last click, effectively-locked layers refuse.
- `selectLayer`, `addLayer`, and every selection-reset path keep
  the set in lock-step with the anchor.

Timeline
- Shift-click on a layer row's name toggles that layer's
  membership in the multi-selection. Every selected row highlights
  via `store.selectedLayerIds.includes(track.layerId)`.
- Right-click on a track now preserves an existing multi-selection
  if the clicked track is part of it; lands as a single-select
  otherwise.
- Track context menu adds two entries when ≥1 layer is selected:
  - "Group N selected layers" — calls the new store action and
    drops the group into the topmost selected track's stack slot
    (not at the very top as a naive `addTrackGroup` would).
  - "Convert N selected to clip" — packs the layers into a fresh
    clip with `duration = scene.duration`, deep-clones via
    `structuredClone(toRaw(l))`, drops a single `ClipInstance` on
    a new clipTrack at `startTime = 0`, and positions the
    clipTrack at the topmost selected track's slot.

Fixes
- `removeLayer` no longer unconditionally clears
  `selectedLayerIds` (bulk-replace artifact from the multi-select
  work); the anchor-matched path clears the whole set, the
  non-anchor path only splices out that one id.
- `deselectIfLocked` indentation normalised.

Regression tests
- Multi-select store API: selectLayer sync, toggleLayerSelection
  add/remove/anchor, lock-filter, empty-selection guards.
- `groupSelectedLayers`: happy-path packing, empty-select null
  return, lock-filter, stack-slot pinning.
- `convertSelectedLayersToClip`: packs layers + spawns clipTrack +
  instance (deep-clone integrity pinned via unique per-layer names
  so the cloned-layer lookup is unambiguous), drops the clipTrack
  at the topmost slot, returns null on empty selection, skips
  effectively-locked layers.

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

src/components/Timeline.vue

@@ -1877,12 +1877,27 @@ async function onClipBlockContextMenu(e: MouseEvent, ci: ClipInstance) {
 const trackMenu = ref<TrackMenuState | null>(null)
 const trackMenuRef = ref<HTMLDivElement | null>(null)
 
+function onLayerTrackNameClick(e: MouseEvent, track: TimelineLayerTrack) {
+  if (isTrackEffectivelyLocked(track)) return
+  if (e.shiftKey && track.isParent) {
+    store.toggleLayerSelection(track.layerId)
+    return
+  }
+  store.selectLayer(track.layerId)
+}
+
 function onTrackContextMenu(e: MouseEvent, layerId: string) {
   e.preventDefault()
   e.stopPropagation()
   const layer = store.activeLayers.find((l) => l.id === layerId)
   if (!layer) return
-  store.selectLayer(layerId)
+  // If the right-clicked layer isn't already in the multi-selection,
+  // treat the right-click as a single-select so the menu actions
+  // operate on what the user actually clicked. Multi-selected ids
+  // persist so "Group selected" has a real list to work with.
+  if (!store.selectedLayerIds.includes(layerId)) {
+    store.selectLayer(layerId)
+  }
   trackMenu.value = {
     kind: 'layer',
     trackId: layerId,
@@ -1959,6 +1974,16 @@ function toggleSeparated() {
   trackMenu.value = null
 }
 
+function groupSelectedFromMenu() {
+  store.groupSelectedLayers()
+  trackMenu.value = null
+}
+
+function convertSelectedToClipFromMenu() {
+  store.convertSelectedLayersToClip()
+  trackMenu.value = null
+}
+
 function deleteTrack() {
   if (!trackMenu.value) return
   if (trackMenu.value.kind === 'layer') {
@@ -2424,12 +2449,13 @@ onBeforeUnmount(() => {
               <span
                 class="track-name"
                 :class="{
-                  selected: track.layerId === store.selectedLayerId && track.isParent,
+                  selected:
+                    track.isParent && store.selectedLayerIds.includes(track.layerId),
                   parent: track.isParent,
                   sub: !track.isParent,
                 }"
                 :draggable="track.isParent && !isTrackEffectivelyLocked(track)"
-                @click="!isTrackEffectivelyLocked(track) ? store.selectLayer(track.layerId) : null"
+                @click="onLayerTrackNameClick($event, track)"
                 @contextmenu="
                   track.isParent && !isTrackEffectivelyLocked(track)
                     ? onTrackContextMenu($event, track.layerId)
@@ -2999,6 +3025,22 @@ onBeforeUnmount(() => {
               class="kf-menu-trailing-check"
             />
           </button>
+          <button
+            v-if="store.selectedLayerIds.length >= 2"
+            class="kf-menu-item"
+            @click="groupSelectedFromMenu"
+          >
+            <PhFolderPlus :size="12" class="kf-menu-icon" />
+            {{ t('timeline.groupSelected', { n: store.selectedLayerIds.length }) }}
+          </button>
+          <button
+            v-if="store.selectedLayerIds.length >= 1"
+            class="kf-menu-item"
+            @click="convertSelectedToClipFromMenu"
+          >
+            <PhFilmStrip :size="12" class="kf-menu-icon" />
+            {{ t('timeline.convertToClip', { n: store.selectedLayerIds.length }) }}
+          </button>
         </template>
       </template>
       <div class="kf-menu-divider"></div>