feat: add per-part CAD model specs

text-to-cad/skills/cad-router/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: cad-router
-description: Route natural-language CAD, mechanical-part, assembly, 3D-printing, and robot-description requests across text-to-cad, SimpleCADAPI, and CADAM, then enforce shared generation, validation, requirement refinement, visual repair, and CAD Viewer handoff. Use for new or modified STEP/STP, SCAD, STL, 3MF, DXF, URDF, SRDF, SDF, or G-code work when the agent must choose the best modeling backend and preserve editable sources.
+description: Route natural-language CAD, mechanical-part, assembly, 3D-printing, and robot-description requests across text-to-cad, SimpleCADAPI, and CADAM; create one editable model-spec.json per part; convert supplied STEP/STP files into exact-base JSON-controlled models; and enforce generation, modification, validation, visual repair, and CAD Viewer handoff. Use for new or modified STEP/STP, SCAD, STL, 3MF, DXF, URDF, SRDF, SDF, or G-code work when the agent must choose a backend, preserve editable sources, edit JSON model parameters, or parameterize an imported STEP.
 ---
 
 # CAD Router
@@ -33,7 +33,13 @@ Choose a backend before modeling, keep its source as the editable authority, and
    - `simplecadapi`: use an installed SimpleCADAPI Skill/runtime; prefer it for gears, racks, ring gears, bearings, cycloidal parts, reducers, replayable graphs, semantic tags, and FreeCAD interchange.
    - `cadam`: generate editable `.scad`, then execute `node scripts/cadam_compile.mjs <source.scad> <output.stl|output.dxf>`. Set `CADAM_ROOT` or pass `--cadam-root` when the CADAM checkout is not beside text-to-cad. Prefer it for fast parametric printable models, repeated patterns, decorative CSG, and parameter controls.
 5. When the route reports `requirement_refinement` or `visual_repair`, refine the brief, generate, execute, inspect multiple views, and repair no more than three visual mismatch rounds before asking the user.
-6. Generate into a task-owned directory and start CAD Viewer with that task directory as `--dir`; do not expose the repository-wide fixture library for a task review. Never change backend during an edit unless conversion is explicitly requested; modify the recorded source of truth.
+6. Generate into a task-owned directory and start CAD Viewer with that task directory as `--dir`; do not expose the repository-wide fixture library for a task review. Write one `model-spec.json` per part and treat it as the editable geometry contract; keep `cad-task.json` as the execution record. Read `references/model-spec.md` before creating, importing, or modifying a model specification. Never change backend during an edit unless conversion is explicitly requested; modify the recorded source of truth.
+   For a supplied STEP/STP without an editable generator, create an
+   `exact_step_base` specification with `scripts/model_spec.py import-step`.
+   Preserve the copied STEP as the immutable exact B-Rep base and express later
+   edits as named JSON parameters and modification features. Do not claim that
+   arbitrary STEP recovers its original sketches, constraints, or feature
+   history.
    For modification, prefer a native Python/SCAD generator. With STEP only,
    inspect the explicitly supplied model and perform the smallest local feature
    rebuild possible. For major topology changes, reconstruct from that model
@@ -67,4 +73,6 @@ Choose a backend before modeling, keep its source as the editable authority, and
 
 - Read `references/capabilities.md` when comparing or invoking backends.
 - Read `references/backend-contract.md` before writing a task manifest or adding another backend.
+- Read `references/model-spec.md` when generating per-part parameter JSON,
+  converting STEP to a JSON-controlled model, or modifying an existing part.
 - `scripts/capabilities.json` is the machine-readable V1 registry used by `scripts/route.py`.