Rebaseline CPU voxel performance

[rsasaki0109]

What changed

• add a dated CPU-only voxel centroid baseline for 10k through 2M XYZI points
• separate current CPU measurements from the historical matched CPU/GPU chart
• document the host, benchmark command, exact medians, rejected optimization probe, and claim boundaries

Why

The README CPU values were stale: the current 2M Criterion median is 47.290 ms versus the dated approximately 389 ms entry. Mixing the new CPU measurement with old GPU data would produce an invalid crossover claim, so the two receipts are now clearly separated.

Validation

• cargo fmt --all --check
• git diff --check
• CPU Criterion benchmark: cargo bench -p spatialrust-filtering --features filter-voxel-gpu --bench voxel_downsample -- cpu_centroid --noplot
• rejected XYZI specialization was parity-tested and fully reverted after regressing 500k–2M medians