D016: Hero uses plain three.js, not R3F

Context: The spec specified React Three Fiber (R3F) for the WebGL Hero with a < ~120KB gz budget. In practice those conflict: R3F v9 dynamic-imports the whole three namespace → measured 241KB gz, double the budget.

Decision: Rewrote the scene as plain three.js with named imports, 129KB gz, same visual, same React lifecycle. Accepted (shipped in Phase 3A - WebGL and Graph).

Why: Honors the "performance is a feature" principle (North Star - Site as Proof) without changing the look. 129KB is marginally over the ~120KB target but is lazy, post-LCP, desktop-only (degraded devices download none of it), so real-world impact is negligible, desktop LCP measured 0.6s.

Open option (Saif's call): the shader is simple enough to run on raw WebGL (~3KB), dropping ~126KB. Recommended if we want the budget met with huge margin; the only cost is a bit more bespoke GL boilerplate and less future scene flexibility (unlikely to matter for one static field). If chosen, update this note.

Trade-offs: plain three = manual scene wiring vs R3F ergonomics; fine for a single field. Supersedes the "R3F" mention in the stack table + WebGL Hero.