0.13.1: new track-group lands above the active layer

addTrackGroup takes an optional anchorTrackId and inserts directly
above it in trackOrder; anchors nested inside another group resolve
to their parent's slot. The timeline's group-add button forwards
selectedLayerId so the new group lands where the user is working
instead of always jumping to the top. Covered by store + UI tests.

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

src/stores/scene.ts

@@ -2660,14 +2660,39 @@ export const useSceneStore = defineStore('scene', {
         else inst.rotation = r
       }
     },
-    addTrackGroup(name?: string): TrackGroup {
+    /** Create an empty group. When `anchorTrackId` is given the group
+     *  lands directly above that track in trackOrder — resolving to
+     *  the parent group's position if the anchor sits inside one. Falls
+     *  back to the top of the stack when the anchor is missing or the
+     *  scene has no trackOrder yet. */
+    addTrackGroup(name?: string, anchorTrackId?: string | null): TrackGroup {
       const group: TrackGroup = {
         id: uid(),
         name: name?.trim() || 'Group',
         childIds: [],
       }
       ;(this.scene.groups ||= []).push(group)
-      ;(this.scene.trackOrder ||= []).unshift(group.id)
+      const order = (this.scene.trackOrder ||= [])
+
+      let insertAt = 0
+      if (anchorTrackId) {
+        const direct = order.indexOf(anchorTrackId)
+        if (direct >= 0) {
+          insertAt = direct
+        } else {
+          // Anchor is inside a group — put the new group above the
+          // group container instead of trying to wedge in between
+          // group children (nested groups are not supported).
+          const parent = this.scene.groups?.find((g) =>
+            g.childIds.includes(anchorTrackId),
+          )
+          if (parent) {
+            const pi = order.indexOf(parent.id)
+            if (pi >= 0) insertAt = pi
+          }
+        }
+      }
+      order.splice(insertAt, 0, group.id)
       return group
     },
     /** Create a new group and move every currently-selected layer