docs(mcp): add 10-agent research on scene-save workflow

10 parallel research agents (R1-R10) investigated whether MCP can save
scenes as saveable entities that the user later opens in the editor —
rather than injecting via a dev-only window.__pascalScene hook.

Key findings:
- Editor is already backend-agnostic via onLoad/onSave callbacks (R2,R7)
- Current persistence is localStorage-only, single key (R1)
- Zero dynamic routes and no backend code yet — env declared (R4,R5)
- File import exists as "Load Build" but lacks Zod validation (R6)
- MCP-written scenes load cleanly into the editor today (R6, Casa del
  Sol test already proved this)
- Best path: filesystem-handoff this week, Supabase in weeks 2-4,
  Supabase Realtime for live mode in Q2. Skip Yjs for now. (R8)
- 4-5 weeks to private beta, 10-14 to GA (R9)
- 10 high-value ideas ranked; "photo -> scene" is the unblocker (R10)

SYNTHESIS.md pulls all 10 reports together with the recommended
implementation plan.

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

packages/mcp/test-reports/research/R6-file-io.md

@@ -0,0 +1,55 @@
+# R6 — File I/O pathways
+
+## TL;DR
+- Export: 2 JSON pathways + 3 binary (GLB/STL/OBJ) pathways.
+- Import: 1 JSON pathway ("Load Build"), **no Zod validation** at boundary.
+- No drag-drop, no clipboard-paste JSON import.
+- Round-trip export → re-import works; MCP-written JSON loads cleanly IF structure matches.
+
+## Exports
+
+| Trigger | Handler | File | Output |
+|---|---|---|---|
+| Settings → "Save Build" | `handleSaveBuild` | `settings-panel/index.tsx:205–216` | `layout_YYYY-MM-DD.json` |
+| Cmd palette → "Export Scene (JSON)" | `editor.export.json` | `command-palette/editor-commands.tsx:329–346` | `scene_YYYY-MM-DD.json` |
+| Settings/palette → "Export GLB/STL/OBJ" | `export-manager.tsx` | `editor/export-manager.tsx:71–78` | binary 3D geometry |
+
+Both JSON paths serialise `{ nodes: useScene.getState().nodes, rootNodeIds: useScene.getState().rootNodeIds }`. No metadata (no name, no created_at, no projectId).
+
+## Import
+
+### "Load Build"
+- `settings-panel/index.tsx:218–239`
+- Accept: `application/json`
+- Handler: `JSON.parse` → check `data.nodes && data.rootNodeIds` → call `useScene.setScene(nodes, rootNodeIds)`
+- **No Zod validation.** Confirmed the security-audit flag from Phase 3.
+
+### `setScene` behaviour (`core/store/use-scene.ts:242–271`)
+1. `migrateNodes()` — runs a few backward-compat patches. Stair nodes are zod-safeParsed and SILENTLY DROPPED on failure; other types are unvalidated.
+2. Orphan pruning — deletes any node whose `parentId` isn't present in the dict.
+3. `setState` with cleaned nodes + `dirtyNodes: new Set()`.
+4. Marks every node dirty to trigger re-render.
+
+**Critical gap:** Invalid `type` strings silently load. Systems will later fail to find a renderer for them and the node will be invisible but consume state.
+
+## Round-trip fidelity
+
+| Scenario | Loads clean? |
+|---|---|
+| Export → re-import | ✅ |
+| MCP writes well-formed `{ nodes, rootNodeIds }` | ✅ (confirmed: Casa del Sol scene.json loads into the editor) |
+| MCP writes bad `node.type` | ⚠️ Silent load, invisible node |
+| MCP writes broken parentId chain | ⚠️ Orphans silently deleted |
+| MCP writes missing `children: []` on container | ⚠️ Core treats as `undefined` → system ignores |
+
+## Missing / nice-to-have
+
+- Drag-drop JSON onto viewport
+- URL-param load: `?load=<publicJsonUrl>`
+- Clipboard paste of JSON blob
+- `importFromFile` with Zod validation at the boundary
+- File-format version field (`formatVersion: "1"`) for forward-compat
+
+## Recommendation
+
+Every "save to cloud" implementation MUST Zod-validate at the boundary with `AnyNode.safeParse` per node + structural checks on `rootNodeIds`. The tool in MCP (`validate_scene`) already does this — run it before any save.