Add M3564C load cell reference part

[BenCaunt]

Summary

• Add a new m3564c-load-cell project with the source PDF, notes, project metadata, and a M3564C six-axis load-cell reference model.
• Move project-owned CAD code out of synthcad/ into project-local packages under projects//, with each project declaring build/URDF targets in targets.py.
• Replace hard-coded flat-disk target registries with project target discovery and project-local generated output defaults.
• Scope CI, tests, exports, reports, and PR diff viewer targets from changed paths via synthcad-select, so project-only changes do not run unrelated project tests or show unrelated CAD.

Validation

• uv run pytest -q
• uv run synthcad-build --list
• uv run synthcad-urdf --list
• uv run synthcad-select --path projects/m3564c-load-cell/tests/test_m3564c_load_cell.py --json
• uv run synthcad-build --target m3564c-six-axis-load-cell --profile
• uv run synthcad-inspect --target m3564c-six-axis-load-cell --interference assemblies --view isometric --view rear-isometric
• uv run synthcad-pr-pages --repo-root . --base-root . --target m3564c-six-axis-load-cell --output-dir /tmp/synthcad-m3564c-pr-site

[github-actions]

SynthCAD CAD review

Compared b5dce19c3768 to 17f2609fe043 for selected targets: flat-disk-robot-chassis flat-disk-robot-lid flat-disk-robot m3564c-six-axis-load-cell.

• Interactive diff viewer
• Workflow run and generated artifacts