[yujiaxu-ops]

Vertex colours, density grids, instance colours, unlit materials and the background used to be @if flags, so every combination of them produced a different kernel source. They are now bits of FEAT, the pipeline-overridable constant that the other scene features already use.

• Their bindings and fields are always declared. The Rust side already provided all of them.
• OBJECT_STRIDE is an override derived from F_TINT. A test runs naga's override processing to check it.
• Only units still change the source. The kernel now has 10 flags instead of 15.

Checks

• cargo test --release with the loop guard: 138 passed.

• render-identity against main, 190 outputs each:

  run	byte-identical	within run-to-run spread	changed
  native + guard	172	18	0
  merged + guard	175	15	0
  uniform tables + guard	171	19	0
  native	172	18	0

• web-hello in Chrome: pass.

• Bistro trace time, alternating with main in both orders:

  • native: 8.39–9.14 ms against main's 8.38–9.23 ms;
  • merged: steady prints 48.4–48.5 ms against 48.5–48.8 ms.

🤖 Generated with Claude Code

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Note

Medium Risk
Changes how scene capability is compiled and how FEAT/OBJECT_STRIDE are resolved at pipeline creation, but identity/render checks reported unchanged output; mistakes could still affect binding layout or stride for tinted scenes.

Overview
Collapses five scene-specific WESL @if kernel variants into one source by representing vertex colours, density grids, instance colours, unlit materials, and scene.background as new FEAT bits (F_TINT, F_VCOLOR, F_GRID, F_UNLIT, F_BACKGROUND), matching how other optional features already work.

The shader always declares the related bindings and struct fields (grid_tex, Params.background, vcolor/vgrid/obj_tint, etc.) and gates behavior at runtime with has(F_*) instead of stripping declarations per variant. OBJECT_STRIDE is now a pipeline override (select(3u, 4u, … F_TINT)); Rust no longer treats tint/vcolor/grid/unlit/background as KernelVariant::flags or F_SOURCE—only units still changes the compiled source (F_SOURCE = F_UNITS), cutting conditional-translation flags from 15 to 10.

Tests and docs were updated: scene_features_are_feat_bits asserts identical WGSL for those feature bitmasks and validates naga override resolution for OBJECT_STRIDE; ROADMAP.md / RUNTIME.md describe the new model.

^{Reviewed by Cursor Bugbot for commit 4cd5e2c. Bugbot is set up for automated code reviews on this repo. Configure here.}