Add precomputed cortical profile cache lookup

Add GT brain render cache, NSD-only phi maps, and parallel precomputation

Add pyvista renderer, precomputed phi map cache, and background-thread steering brain renders

Comment out new features (CLIP×φ sort, λ input, brain MEI sidecar) until ready

Sort by CLIP×φ, per-feature λ input, brain MEI sidecar support, gallery in UI

Switch to modular explorer package (scripts/explorer/)