Vulkan Forward+: luminance-priority tile lists when overloaded

[timfox]

Summary

Implements roadmap item from FORWARD_PLUS_PIPELINE_AUDIT.md §8: when a 16px tile has more overlapping lights than r_forwardPlusMaxPerTile, the tile cull now keeps the top N by RGB luminance sum (packed dlight color) instead of always taking the first N light indices.

New cvar

• r_forwardPlusLuminanceSort — 0 / 1, default 1, CVAR_ARCHIVE_ND. 1 = partial selection by RGB sum when overloaded; 0 = legacy first-candidate order when overloaded. No vid_restart (push constant per dispatch). Startup log when Forward+ is on and sort is 1.

Implementation

• forward_plus_tile_cull.comp: per-tile candidate list (max 32), then partial selection sort for first maxPerTile rows when candCount > cap and luminanceSort != 0.
• vk_forward_plus.c: vk_fp_push_t extended with luminance_sort uint; pipeline layout push range size updated; dispatch fills from cvar.
• SPIR-V regenerated via ./scripts/compile_shaders.sh --apply (shader_data.c + generated tree).

Docs

• docs/FORWARD_PLUS_PIPELINE_AUDIT.md, docs/QUICKSTART.md, CHANGELOG.md.

Validation

• ninja -C build-vk-Release (Vulkan renderer)
• bash scripts/renderer_regression_check.sh — passed

Notes

• Under capacity, tile output order remains increasing light index (same as before).
• Sort key is linear RGB sum from SSBO color vec4 — cheap heuristic, not distance-based (still listed as future work in audit).