feat: integrate SimpleCADAPI 2.0.2 CAD workflows

text-to-cad/skills/cad-router/references/designir-3.0.md

@@ -113,6 +113,49 @@ The adapter loads only formally promoted experiences whose scope is
 applied conditionally by the runtime; `cad-task.json` records both the available
 method count and the exact method IDs used for the task.
 
+## Compact single-file storage
+
+A DesignIR remains one source-independent JSON file. For surface-parametric and
+hybrid documents, the dense `surface_layer` may use the lossless
+`gzip+base64+json` storage envelope:
+
+```json
+{
+  "surface_layer": {
+    "format": "surfaceir-3.0",
+    "encoding": "gzip+base64+json",
+    "uncompressed_bytes": 10286354,
+    "compressed_bytes": 2358852,
+    "sha256": "<digest>",
+    "data": "<base64>"
+  }
+}
+```
+
+The runtime verifies the byte counts and SHA-256 digest before rebuilding.
+Decoding restores the same SurfaceIR object; no coordinate, topology, pcurve,
+tolerance, parameter binding, or edit information is discarded. Legacy
+documents containing an expanded `surface_layer` remain supported.
+
+Agents must read the ordinary top-level `semantic_layer`, `edit_interface`, and
+validated `parameters.json`. They must not place the encoded `surface_layer.data`
+string in an LLM prompt. Deterministic CAD runtimes alone decode that field.
+
+## Lazy URDF and MJCF export
+
+URDF and MJCF are derived artifacts, not additional DesignIR authorities.
+Generate them only after an explicit download/export request. The deterministic
+exporter rebuilds from DesignIR alone and packages:
+
+- the `.urdf` or `.mjcf.xml` description;
+- an STL mesh used for visual and collision geometry;
+- `robot-export.json` with source hash, scale, reconstruction strategy,
+  structural assumptions, and dynamics status.
+
+STEP-derived geometry without authoritative assembly, joint, or material
+evidence exports as one fixed `base_link`. Do not infer articulated joints,
+limits, actuators, density, mass, or inertia merely from disconnected solids.
+
 ## Parameter boundary
 
 STEP does not contain authoritative feature names or modeling history. The