fix(vitals): load Soma URDF mesh assets

[LittleRookie1115]

Problem

The Vitals client could receive URDF text from Soma, but its local wire schema
did not include attached URDF assets. Relative mesh references therefore had no
browser-accessible base URL, causing mesh-based robot models to render without
their visual geometry.

Changes

• Add the additive UrdfAsset, include_assets, and assets wire fields.
• Request referenced assets when loading the Soma URDF.
• Store assets in the existing content-addressed in-memory cache.
• Expose the generated immutable asset base URL to the Vitals renderer.
• Preserve compatibility with Soma responses that do not include assets.
• Add wire compatibility, asset staging, and fallback tests.

Validation

• .venv/bin/python -m unittest discover -s tests -v — 41 passed.
• .venv/bin/python -m unittest tests.test_vitals_transport tests.test_urdf_assets -v — 21 passed.
• .venv/bin/python -m pip check — passed.
• npm ci — passed with no vulnerabilities.
• npm run build — passed; committed bundle remained unchanged.
• npm run test:visual — 16 passed.
• python3 scripts/check_commit_authorship.py --base upstream/main --head HEAD — passed.

Compatibility

The protobuf changes are additive. Older Soma deployments that return no
assets continue to load the URDF with an empty asset base URL. No database or
configuration migration is required.

Related change

• Backend URDF asset support: feat(vitals): add robot health monitoring and visual models robonix#219.