[codex] add demo ftc crayon plan

[BenCaunt]

Summary

Adds a CrayonPlan-style robot planning workflow and a new demo-ftc-robot concept project for the FTC DECODE game.

The new concept assembly includes a 16 in planning envelope, six-wheel drive, three stored DECODE artifacts, a front-facing intake, a second-stage indexer, and a front-facing 72 mm flywheel shooter with two 1 in hood rollers on the lid.

The change also keeps ownership boundaries explicit: generic planning primitives live in synthcad/library/crayon.py, while FTC-specific proxies such as the Yellow Jacket motor envelope live in synthcad/projects/demo_ftc_robot/parts.py.

Viewer and Artifacts

• Records per-child color metadata in display snapshots.
• Shows color swatches in the local viewer and PR diff viewer part lists.
• Teaches build/viewer paths to use project-local generated directories for non-default projects.

Validation

• uv run synthcad-build --project demo-ftc-robot
• uv run synthcad-view --project demo-ftc-robot --list
• Full test suite via the local readline workaround: 51 passed

Note: plain uv run pytest still hits the local Python readline segfault in this environment, so tests were run by preloading a stub readline module.

[github-actions]

SynthCAD CAD review

Compared 50cc8c6e6505 to dcdc11e032f5 for demo-ftc-robot.

• Interactive diff viewer
• Workflow run and generated artifacts