Harden multimodal CAD planning and verification

[SmartAI]

Summary

• extend the CAD verification gate with wall-thickness checks, anchored holes, and targeted symmetry
• enforce evidence-backed plans and spec-sheet continuity for image-driven CAD requests
• add local build123d documentation search with retrieval tests and harden the agent prompt for text and image inputs

Validation

• npm run typecheck
• npm test (370 tests)
• focused system-prompt tests

Notes

• the prompt follows OpenAI GPT-5.6 outcome-first guidance while preserving Chamfer runtime and verification invariants
• the RST parser test intentionally contains an underline made of equals signs, which git diff --check recognizes as a conflict-marker-like fixture