Three.js: build and verify runtime harness before promoting from planned

[islee23520]

Source

• README.md and docs/usage.md mark Three.js as planned.
• docs/usage.md says runtime harness is absent in this repo snapshot.

Current state

Adapter files may exist under bridge/threejs, but runtime support must not be presented as verified behavior.

Acceptance criteria

• A Three.js runtime harness exists and is exercised through a supported Lux surface.
• Project detection, .lux workspace initialization, bridge install/status, build/run/test, and workflow skill projection each have explicit maturity status.
• Verification records concrete command output and evidence under .lux/.
• Docs and CLI/API/MCP projections keep Three.js planned until evidence justifies an upgrade.