v2.1: Improve performance by ~35 times

[mourner]

Performance-focused rewrite of the core distance computation:

• Edge-block bbox skip: group each ring's edges into blocks of 32 consecutive edges and precompute one bounding box per block (a single flat Float64Array, sized upfront; block layout is re-derived in the scan from coords/ringEnds, so no auxiliary index arrays). In the per-cell distance scan, a cheap point-to-bbox lower bound skips a whole block in O(1) when it can neither beat the current nearest distance nor flip the ray-cast parity. Polygon rings are spatially coherent, so most blocks are far from any given cell center and get skipped wholesale.
• Flattened coordinate buffer. Rings are copied once into a single contiguous Float64Array (coords) with per-ring end offsets (ringEnds), replacing nested array-of-[x, y] iteration. This removes pointer-chasing in the hot distance loop for cache-friendly access.
• Early-exit distance scan. pointToPolygonDist now takes a distance threshold (maxD). A cell is only useful if it can beat the current best or is worth subdividing; once the center is provably within the threshold of the outline, the scan bails out early instead of measuring the whole outline.
• Nearest-segment seeding. Each cell records the nearest segment found during its scan. When a cell is split, its four children seed their scan with the parent's nearest segment — a child is almost always nearest to the same segment, so boundary cells hit the early-out threshold immediately.