feat(octree near): occupancy-bucketed compact W-tile near-field builder

[TobiBu]

Summary

Adds octree_nearfield_tiles.build_octree_nearfield_tiles — a static-shape builder that turns an adaptive octree's occupancy-bounded near CSR into occupancy-bucketed, padding-free W-tiles for the near-field P2P.

Each adaptive cell is re-packed into full uniform width-W tiles (cell-respecting, so tile-pairs map 1:1 onto cell-pairs — no cross-cell masking, no double-counting), and each tile's source-tile list is densely compacted to a capacity sized by the actual near work rather than the (1 + u_capacity) * ceil(max_leaf/W) worst case. Shapes depend only on (num_tiles_cap, W, max_source_tiles), so it compiles once and is reused across timesteps; overflow is a single eager-checked boolean.

Consumed by jaccpot's octree near field as the near_mode="compact_wtile" structure (separate jaccpot PR).

Validation (tests/unit/test_octree_nearfield_tiles.py, 100% module coverage)

• Coverage / cell-respecting: every particle in exactly one tile; every tile confined to a single leaf cell.
• Source-tile completeness: each target tile's compacted source set == the reference leaf-near expansion (own cell's other tiles + U-neighbour cells' tiles), self-excluded.
• Overflow flag trips exactly when a cap is under-sized.

🤖 Generated with Claude Code