Parallelize bounded CPU FPFH descriptor phases

[rsasaki0109]

FPFH computed every SPFH row and weighted descriptor on the caller despite independent point work in each phase. Share one caller-built KD-tree across bounded scoped CPU workers, writing disjoint contiguous slices directly into the existing descriptor arrays. Join all SPFH workers before weighting reads their rows. Reuse existing point staging; small/one-worker clouds stay serial.

Each point keeps the unsorted radius-neighbor order, Darboux/bin/normalization math and 33-bin f32 accumulation order. Matching, seeded RANSAC and validation retain their existing execution. Public signatures and dependencies are unchanged. Scratch remains neighborhood-dependent per worker; concurrent calls have their own budgets.

Validation:

• Five new tests compare 18,402,648 exact descriptor words against the frozen serial public API with forced 1/2/6 workers and public staging, including duplicate/curved/planar inputs and 32,771-point uneven bands. Independent planar/degenerate expectations, invalid inputs, concurrency and finite/converged seeded alignment are covered.
• 19 minimal and 35 all-feature registration tests pass, plus the minimal facade smoke. Strict minimal/all-feature Clippy, facade Clippy, crate/facade WASM compilation, formatting and diff checks pass. WASM execution is not claimed.
• Twelve release completed-API conditions, two warmups and twenty rotating old/forced-serial/public rounds each; every output matches frozen descriptor bits. Timers include validation, lifting, KD-tree build, both allocations/phases, worker spawn/join and intermediate destruction.
• Observed old/public ratios are 1.421–1.458 at 32,768 points/two workers and 2.375–2.595 at 65,536/four. All six multi-worker intervals are above one. One-worker public controls range 0.984–1.000, with small regressions; all controls and ranges are recorded without a portable scaling guarantee.

Implementation and full measurement note.

Based on merged PR #144 (main f83ed25). Merge is gated on the frozen source head, all 114 named checks succeeding, no pending/failed checks and a clean merge state.