Rework neighbors API

[tychedelia]

Neighbor lists as per-particle attributes, like TouchDesigner's Neighbor POP.

• FIND_NEIGHBORS searches the grid once and writes neighbors (up to max indices, default 64) and neighbor_count onto each particle. Past max it keeps a spread sample rather than the nearest.
• NEIGHBOR reads those lists, so several reductions share one search (flocking does five). New: relative=True for a_j - a_i, and an INVERSE_SQUARE falloff. Particles never count themselves. grid= still does an exact search.
• Custom kernels just declare neighbors and neighbor_count, and apply binds them by name like position.

  p.apply(FIND_NEIGHBORS, grid=grid, radius=2.5)
  p.apply(NEIGHBOR, "velocity", out="alignment", op=SUM)
  p.apply(NEIGHBOR, "position", out="separation", op=SUM, relative=True, radius=1.2, falloff=INVERSE_SQUARE)

Also adds processing_particles_find_neighbors to the FFI.

[github-actions]

Visual regression: 43 new of 43 cases

case	status	detail
rectangle	🆕 new	no baseline on main
primitives_3d	🆕 new	no baseline on main
particles_noise	🆕 new	no baseline on main
py_primitives_3d	🆕 new	no baseline on main
py_particles_noise	🆕 new	no baseline on main
py_animated_mesh	🆕 new	no baseline on main
py_background_image	🆕 new	no baseline on main
py_blend_modes	🆕 new	no baseline on main
py_box	🆕 new	no baseline on main
py_camera_controllers	🆕 new	no baseline on main
py_curves	🆕 new	no baseline on main
py_custom_material	🆕 new	no baseline on main
py_feedback	🆕 new	no baseline on main
py_flocking	🆕 new	no baseline on main
py_flocking_duck	🆕 new	no baseline on main
py_flocking_gpu	🆕 new	no baseline on main
py_geometry_methods	🆕 new	no baseline on main
py_gltf_load	🆕 new	no baseline on main
py_lights	🆕 new	no baseline on main
py_materials	🆕 new	no baseline on main
py_multi_window	🆕 new	no baseline on main
py_particles_animated	🆕 new	no baseline on main
py_particles_basic	🆕 new	no baseline on main
py_particles_density	🆕 new	no baseline on main
py_particles_emit	🆕 new	no baseline on main
py_particles_emit_gpu	🆕 new	no baseline on main
py_particles_from_mesh	🆕 new	no baseline on main
py_particles_gpu_surface	🆕 new	no baseline on main
py_particles_gpu_surface_lit	🆕 new	no baseline on main
py_particles_lifecycle	🆕 new	no baseline on main
py_particles_lines	🆕 new	no baseline on main
py_particles_lissajous	🆕 new	no baseline on main
py_particles_points	🆕 new	no baseline on main
py_particles_scatter_volume	🆕 new	no baseline on main
py_particles_sphere	🆕 new	no baseline on main
py_particles_stress	🆕 new	no baseline on main
py_particles_surface	🆕 new	no baseline on main
py_primitives_2d	🆕 new	no baseline on main
py_rectangle	🆕 new	no baseline on main
py_shapes	🆕 new	no baseline on main
py_style_stack	🆕 new	no baseline on main
py_text	🆕 new	no baseline on main
py_window_controls	🆕 new	no baseline on main

_{baseline unknown · adapter llvmpipe (LLVM 20.1.2, 256 bits) (Vulkan, Mesa 25.2.8-0ubuntu0.24.04.2 (LLVM 20.1.2))}

Open the visual report · workflow run