feat: strengthen CAD experience distillation and routing

text-to-cad/skills/cad-router/references/model-spec.md

@@ -68,7 +68,8 @@ constraints, names, or feature history. Therefore:
 ## Parametric modification layer
 
 Supported V1 operations are `add_box`, `cut_box`, `add_cylinder`, and
-`cut_cylinder`. Values may be literals or parameter references:
+`cut_cylinder`. Values may be literals, direct parameter references, or affine
+parameter expressions:
 
 ```json
 {
@@ -82,6 +83,21 @@ Supported V1 operations are `add_box`, `cut_box`, `add_cylinder`, and
 }
 ```
 
+Use `scale` and `offset` when a tool center must stay anchored while its size
+changes. For example, a positive-Z face at `z = 14.1` can remain the lower face
+of an added box while `extension` changes:
+
+```json
+{
+  "center": [
+    13.1,
+    -1.0,
+    {"parameter": "extension", "scale": 0.5, "offset": 14.1}
+  ],
+  "size": [22.2, 2.0, {"parameter": "extension"}]
+}
+```
+
 Change a named value and rebuild:
 
 ```bash