Global illumination by voxel cone tracing

Every glowing texel lights the world through a voxel grid; walls stop light, windows let it out, interiors are lit by their fires and windows, the sky only where it is seen. Cached irradiance probes are traced again only where something changed (docs/lighting-gi.md).

Built on feat/vct-gi: bricks from sprites, clipmap with per-block re-voxelization, relight / downsample / trace / invalidate compute passes, probe lookup with a visibility test in the impostor, terrain and mesh shaders, quality setting off / low / medium / high / ultra, debug views (I).

Merged with the GPU-timed budget (work around the player's storey first), calibrated glow (twice the old point lights), the night floor above the toon black threshold and floors that block light. The long read How Felucca draws a pixel explains it with captures.

Later: flames flickering in the GI (animate the emission of a few hot blocks), smoother probe updates (blend old and new), measurements on weaker GPUs.