feat: embed surface — export in-canvas affordances + transparent/faint-ink viewer support

Lets a host mount the editor's real editing experience on a bare <Viewer>
without the full <Editor> shell.

editor: export Grid, NodeArrowHandles, MoveTool, ToolManager — the selection
handles, the kind-owned mover, the build-tool host, and the drafting grid that
feeds tools their grid:* pointer events. See the doc comment in index.tsx for
how they cooperate with host camera controls and selection.

viewer: two opt-in, non-persisted presentation flags (both default off, so the
editor and every other consumer are unchanged):
- transparentBackground / <Viewer transparent>: emit premultiplied RGBA masked
  by geometry + outline alpha (outputColorTransform off on that path) so the
  scene can float on any page background. ACES tone-mapping makes a true-white
  opaque background impossible, hence transparency.
- inkOpacity: override the per-mode ink-edge opacity.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/viewer/src/store/use-viewer.ts

@@ -75,6 +75,13 @@ type ViewerState = {
   showGrid: boolean
   setShowGrid: (show: boolean) => void
 
+  transparentBackground: boolean
+  setTransparentBackground: (transparent: boolean) => void
+
+  // Embed-controlled ink-edge opacity override (null = use the per-mode default).
+  inkOpacity: number | null
+  setInkOpacity: (opacity: number | null) => void
+
   projectId: string | null
   setProjectId: (id: string | null) => void
   projectPreferences: Record<
@@ -283,6 +290,12 @@ const useViewer = create<ViewerState>()(
           return { showGrid: show, projectPreferences }
         }),
 
+      transparentBackground: false,
+      setTransparentBackground: (transparent) => set({ transparentBackground: transparent }),
+
+      inkOpacity: null,
+      setInkOpacity: (opacity) => set({ inkOpacity: opacity }),
+
       projectId: null,
       setProjectId: (id) =>
         set((state) => {