ci: run host-neutral engine tests on Ubuntu

[jeffglousher]

Summary

• Adds a 47-line ubuntu-24.04 job that runs cargo test --locked for nbcad-core, nbcad-sketch, nbcad-assembly, and nbcad-solid.
• Fills the Linux gap: desktop-packages.yml only builds Windows/macOS, and mcp-server.yml only tests on windows-latest. This is not a Linux desktop/Bevy package.
• Branched from jackControls/noBS-CAD main (351cef6); one commit, no fork stack.

Test plan

• cargo test --locked -p nbcad-core -p nbcad-sketch -p nbcad-assembly -p nbcad-solid passed on Linux 6.12 (this box) against PR fix: scale perpendicular residuals so chained right-angle drags converge #50 HEAD
• Confirm this workflow is green on the PR check
• Confirm path filters skip the job when only desktop/docs change

[jackControls]

I am working on a Ubuntu support branch. Let's merge this after once that's ready.