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/editor/src/index.tsx

@@ -12,12 +12,34 @@ export { default as Editor } from './components/editor'
 // surface uses the shorter, shell-friendly names from the unified
 // preset-system spec.
 export { FloatingActionMenu as FloatingMenu } from './components/editor/floating-action-menu'
+// Embed surface — the editor's real in-canvas affordances, so a host can mount
+// authentic selection handles, interactive build tools, and the mover on top
+// of a bare `<Viewer>` without the full `<Editor>` shell.
+//  - `NodeArrowHandles` renders the selected node's registry resize/rotate/move
+//    handles.
+//  - `MoveTool` runs the kind-owned mover once a translate handle arms
+//    `useEditor.movingNode`.
+//  - `ToolManager` mounts the active registry build tool (wall / door / window /
+//    …) for interactive placement when `useEditor` is in build mode with a
+//    tool, plus the snap/alignment guide layers. Mount it only while a tool is
+//    active to avoid its select-mode boundary editors.
+//  - `Grid` is the interactive drafting plane: it raycasts the pointer and
+//    emits the `grid:move` / `grid:click` events the build tools consume (the
+//    wall tool is driven entirely by them; door/window use them for free-follow
+//    alongside the viewer's `wall:*` mesh events). Without it the tools mount
+//    but their cursor never tracks the pointer. Mount it while a tool is active.
+// All read `useViewer` selection + `useEditor` state, and cooperate with host
+// camera controls via the `useViewer.inputDragging` / `useEditor.movingNode`
+// flags. Tools place onto `useViewer.selection.levelId`, so the host must set a
+// building + level selection first.
+export { Grid } from './components/editor/grid'
 export {
   DimensionPill,
   type DimensionPillPart,
   formatMeasurement,
   MeasurementPill,
 } from './components/editor/measurement-pill'
+export { NodeArrowHandles } from './components/editor/node-arrow-handles'
 export {
   type SnapshotCameraData,
   ThumbnailGenerator,
@@ -40,6 +62,7 @@ export {
   type FencePlanPoint,
   snapFenceDraftPoint,
 } from './components/tools/fence/fence-drafting'
+export { MoveTool } from './components/tools/item/move-tool'
 // Placement-math helpers — shared by kind-owned placement tools in
 // `@pascal-app/nodes` (wall curve sagitta snap, door / window placement,
 // item drop) so kinds don't reach into editor internals.
@@ -101,6 +124,7 @@ export {
   DEFAULT_STAIR_TYPE,
   DEFAULT_STAIR_WIDTH,
 } from './components/tools/stair/stair-defaults'
+export { ToolManager } from './components/tools/tool-manager'
 export {
   createWallOnCurrentLevel,
   getSegmentGridStep,