Function simplifyForCapsule

Simplify voxel collision data for upright capsule navigation.

Uses bitfield storage (1 bit per voxel) to reduce memory by 8x compared to byte-per-voxel. Two Uint32Array buffers are ping-ponged through the dilation, BFS, inversion, and erosion phases.

Algorithm:

1. Build dense bitfield grid from the accumulator.
2. Dilate solid by the capsule shape (Minkowski sum) to get the clearance grid -- cells where the capsule center cannot be placed.
3. BFS flood fill from the seed through free (non-blocked) cells to find all reachable capsule-center positions (uses a separate visited bitfield).
4. Invert: every non-reachable cell becomes solid (negative space carving), computed as a single bitwise operation per word.
5. Erode the solid by the capsule shape (Minkowski subtraction) to shrink surfaces back to their original positions, undoing the inflation from step 2 so the runtime capsule query produces correct collisions.
6. Crop to bounding box of navigable cells.

The flood fill is bounded by the finite grid extents: out-of-bounds cells are never visited, but grid boundaries are not explicitly modeled as solid. This means unsealed scenes may allow navigation up to the edge of the grid.