Add WallCraft AI MVP scaffolding and documentation

api-contract.md

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+Cloud AI Endpoint Contract (example)
+
+Endpoint
+- POST https://api.youraihost.com/ai/map-prep
+
+Headers
+- Authorization: Bearer <apiToken>
+- Content-Type: application/json
+
+Request body (see data-contracts.md)
+- {
+  "scene": { "id": "...", "name": "..." },
+  "gridSize": 32,
+  "walls": [...],
+  "tokens": [...],
+  "promptTemplate": "...",
+  "providerMetadata": { "providerName": "...", "version": "..." },
+  "previewOnly": true
+  }
+
+Response
+- 200 OK
+  - {
+      "walls": [...],
+      "notes": ["Preview generated"],
+      "validationHints": [...],
+      "previewOnly": true
+    }
+- 4xx/5xx responses
+  - { "error": "invalid_payload", "message": "..." }
+  - { "error": "auth_failed", "message": "Invalid API token" }
+
+Security and Privacy
+- Use HTTPS; support short-lived tokens
+- Minimize data; avoid sending unrelated scene data unless required
+- Log access on the server side; provide per-request trace IDs