feat(editor): stage overlay slot + pro capture overlay for studio (#465)

* feat(editor): stage overlay slot, preselectable snapshot crop, pro capture overlay

- Editor/EditorLayoutV2 gain a stageOverlay slot rendered over the canvas
  but under the viewer toolbar (z-10 < z-20), so hosts can swap in a
  full-stage surface (e.g. studio gallery) without unmounting the WebGL
  canvas; FloatingLevelSelector hides while a stage overlay is active
- CaptureMode 'standard' accepts an optional crop preselection
  (SnapshotCropMode, exported) so hosts can enter capture with
  standard/viewport/area already chosen
- SnapshotCaptureOverlay restyle: letterboxed 16:9 frame with corner
  accents + rule-of-thirds for standard mode, thirds grid in viewport
  and area modes, top-center crop/format HUD chips, shutter button with
  capture/saved states — all existing crop, drag, resize and preset
  behaviors preserved

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(editor): standard-capture aspect presets, capture scrim, caption fix

- Standard crop gains aspect presets (16:9, 9:16, 4:3, 3:4, 1:1) — clicking
  the active Standard chip opens the picker; the letterbox frame, HUD and
  output resolution follow the choice
- ThumbnailGenerateEvent carries standardSize; ThumbnailGenerator
  center-crops to it (default stays 1920×1080)
- Subtle bottom scrim keeps shutter/caption readable on bright scenes
- Fix missing space in the snapshot caption

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(editor): standard-capture aspect preselect via CaptureMode

Hosts (the studio capbar) can now pass standardAspect alongside crop when
entering capture mode; the overlay initializes its aspect picker from it.
SnapshotStandardAspect exported.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/core/src/events/bus.ts

@@ -156,6 +156,11 @@ export interface ThumbnailGenerateEvent {
   projectId: string
   captureMode?: 'standard' | 'viewport' | 'area'
   cropRegion?: { x: number; y: number; width: number; height: number }
+  /**
+   * Output size for `standard` captures (center-crop target). Defaults to
+   * 1920×1080; the capture overlay passes other aspect presets (9:16, 4:3…).
+   */
+  standardSize?: { w: number; h: number }
   /**
    * When true, snap levels to their true positions before capturing (for a
    * consistent auto-thumbnail angle) and defer the capture if the tab is