Fix Quake floor grounding fallback

[agustin-lowpoly]

Summary

• fall back to the prepared static floor grid when hull floor tracing misses a walkable brush
• add regression coverage for the E1M4 copied-view stuck pose
• sample fallback floor cells across all prepared shareware maps and verify short grounded moves stay grounded

Validation

• git diff --cached --check
• node --test test/collisionFloor.test.mjs test/movementConstants.test.mjs
• node --test test/*.test.mjs
• pnpm build