feat(mcp,editor): Option A+B storage + 10 agent deliverables (Phase 7)

Ships the combined filesystem/Supabase storage adapter + MCP scene
lifecycle tools + Next.js API routes + editor /scene/[id] route, so
an MCP save is directly openable at /scene/<id> without any
injection hack. End-to-end verified: 10/10 e2e steps pass.

Storage (A1/A2/A3):
- SceneStore interface + error classes + slug helpers
- FilesystemSceneStore at $PASCAL_DATA_DIR (defaults XDG/~/.pascal)
  with atomic writes, .index sidecar, optimistic locking
- SupabaseSceneStore with scenes + scene_revisions tables, RLS
  migration SQL, mock-backed unit tests
- createSceneStore(env) auto-selects based on SUPABASE_URL +
  SUPABASE_SERVICE_ROLE_KEY

MCP tools (A4, A8, A9, A10):
- save_scene / load_scene / list_scenes / delete_scene / rename_scene
- list_templates / create_from_template (3 seed templates:
  empty-studio, two-bedroom, garden-house)
- generate_variants (7 mutation kinds, seeded RNG, save=true|false)
- photo_to_scene (vision sampling → scene graph → save)

Editor (A5, A6):
- /api/scenes + /api/scenes/[id] with RFC 7232 If-Match locking
- /scene/[id] and /scenes route pages with save button, SceneLoader
- Removed the window.__pascalScene dev injection hack

Security + UX edges (A7, A8):
- AssetUrl Zod validator: asset:// blob: data:image/ /path https:
  (http://localhost for dev) + PASCAL_ALLOWED_ASSET_ORIGINS env
  allowlist. Hardens scan.url, guide.url, item.asset.src,
  material.texture.url, MaterialMaps.*Map
- Auto-frame camera on empty→non-empty scene transition
  (camera-controls:fit-scene emitter event)

Shared utilities:
- rehydrateSiteChildren() extracted to packages/mcp/src/lib/ and
  used by both create-from-template and generate-variants to work
  around the SiteNode.children-as-objects vs. ids inconsistency
  (CROSS_CUTTING §2)
- Storage + MCP subpath exports added to packages/mcp/package.json
  (CROSS_CUTTING §4)

Tests: 293 pass / 0 fail across 40 files (was 142 pre-Phase-7).
Biome: clean.

Phase-7 e2e script at packages/mcp/test-reports/phase7-e2e.ts:
MCP HTTP + editor Next.js both point at $PASCAL_DATA_DIR =
/tmp/pascal-e2e, save_scene from MCP, GET /api/scenes/<id> from
editor server, /scenes list page renders all saved scenes, scene
page renders SceneLoader, delete_scene works.

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

packages/mcp/src/lib/rehydrate-site-children.ts

@@ -0,0 +1,40 @@
+import type { AnyNode } from '@pascal-app/core/schema'
+import type { SceneGraph } from '@pascal-app/core/clone-scene-graph'
+
+/**
+ * `cloneSceneGraph` normalises `SiteNode.children` to an array of node IDs,
+ * but core's `SiteNode` schema expects an array of embedded `BuildingNode` /
+ * `ItemNode` objects (see `packages/mcp/CROSS_CUTTING.md` §2). To keep the
+ * cloned graph validating against `AnyNode`, re-embed the site children from
+ * the flat dict.
+ *
+ * Pure: returns a new graph without mutating the input.
+ */
+export function rehydrateSiteChildren(graph: SceneGraph): SceneGraph {
+  const out: SceneGraph = {
+    nodes: { ...graph.nodes },
+    rootNodeIds: [...graph.rootNodeIds],
+    ...(graph.collections ? { collections: graph.collections } : {}),
+  }
+  for (const [id, node] of Object.entries(out.nodes)) {
+    if (node.type !== 'site') continue
+    const childrenField = (node as { children?: unknown[] }).children
+    if (!Array.isArray(childrenField)) continue
+    const rehydrated: AnyNode[] = []
+    for (const child of childrenField) {
+      if (typeof child === 'string') {
+        const target = out.nodes[child as keyof typeof out.nodes]
+        if (target && (target.type === 'building' || target.type === 'item')) {
+          rehydrated.push(target)
+        }
+      } else if (child && typeof child === 'object' && 'id' in (child as Record<string, unknown>)) {
+        rehydrated.push(child as AnyNode)
+      }
+    }
+    out.nodes[id as keyof typeof out.nodes] = {
+      ...(node as AnyNode),
+      children: rehydrated,
+    } as AnyNode
+  }
+  return out
+}