Projects an XY point onto a cell's floor polygons via brute-force
triangle iteration + barycentric Z interpolation. Fan-triangulates
quads and larger polygons. Returns null when outside all floor
surfaces. Accepts pre-transformed world-space vertex positions so
the caller handles EnvCell coordinate transforms.
Second component of the physics collision engine.
4 new tests, all green.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
