Low-res IDW of dot colours (72x46 Gaussian-weighted, σ≈3.1), desaturated 50% and
faded by local density, bilinear-upscaled behind the dots → pink kick region,
blue keys region, purple bass / yellow hat pockets emerge as a slow gradient.
Cached offscreen, rebuilt only when colours/axes/filter change (never on hover);
'regions' toggle in the controls; auto-off in vibe-search mode.