Add WallCraft AI MVP scaffolding and documentation

data-contracts.md

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+AI Prep Data Contracts (Input to AI, and AI Output)
+
+Input payload to cloud AI
+- Schema (JSON)
+  {
+    "scene": { "id": "scene123", "name": "Test Scene" },
+    "gridSize": 32,
+    "walls": [
+      { "c": [1,1,5,1], "type": "wall", "notes": "outer boundary" },
+      { "c": [5,1,5,5], "type": "door", "notes": "entry" }
+    ],
+    "tokens": [
+      { "id": "tok1", "x": 3, "y": 3, "w": 1, "h": 1 }
+    ],
+    "promptTemplate": "Fixed MVP prompt template string",
+    "providerMetadata": { "providerName": "OpenCodeCloud", "version": "v1" },
+    "previewOnly": true
+  }
+
+AI output payload
+- Schema (JSON)
+  {
+    "walls": [
+      { "c": [1,1,6,1], "type": "wall", "notes": "perimeter suggestion" },
+      { "c": [6,1,6,6], "type": "door", "notes": "future entry" }
+    ],
+    "notes": [ "Preview generated" ],
+    "validationHints": [ "Walls may cross token area; adjust endpoints" ],
+    "previewOnly": true
+  }
+
+Notes
+- All coordinates are grid-aligned integers.
+- Unknown fields from provider can be ignored by the client.
+- If previewOnly is true, apply is disabled until user explicitly enables it.