Technical documentation: Vulkan backlog, TRELLIS, Q3/OA compat checks

[cursor]

Summary

Keeps renderer and runtime documentation aligned with recent Vulkan work (Forward+ depth cull, VDB fog binding, RTX entity TLAS), the TRELLIS.2 FLUX-style pipeline, and new static compatibility checks—without inventing behavior beyond tr_init.c, shaders, and the referenced scripts.

Docs added/updated

• docs/FORWARD_PLUS_PIPELINE_AUDIT.md — r_forwardPlusDepthCull, post-opaque dispatch, compute depthCull path, barriers vs main-pass depth.
• docs/RENDERERS.md — Forward+ cvar table (r_forwardPlusDistanceSort, r_forwardPlusDepthCull); corrected r_forwardPlus default (1 on Vulkan) to match tr_init.c.
• docs/RENDERERS_FUTURE.md — RTX demo status: r_rtxEntities, entity cap, two-instance TLAS.
• docs/VOLUMETRIC_FOG_ENHANCEMENTS.md — r_vdbFog / r_vdbFogBlend and volumetric_fog.comp binding 17 behavior.
• docs/TRELLIS.md — commands, cvars, build flag USE_TRELLIS, optional headless scripts.
• docs/TODO_TRIAGE.md, docs/ARCHITECTURE.md — small cross-links or wording as in branch diff.
• docs/RENDERER_CONFIDENCE.md — documents scripts/q3_openarena_compat_check.sh (invoked from smoke_test.sh / validate_ci_build.sh); Forward+ note reflects default-on Vulkan.
• CHANGELOG.md — Forward+ depth cull, RTX entities, VDB fog.
• AGENTS.md — TRELLIS one-line pointer.

Codepaths covered

• Vulkan Forward+ (vk_forward_plus.c, forward_plus_tile_cull.comp, tr_backend.c, tr_shade.c).
• Volumetric / VDB (vk_vdb.c, volumetric_fog.comp, volumetric params).
• RTX demo path (vk_rtx.c, vk_rtx_entities.c).
• Client generative orchestration (cl_main.c) and scripts/trellis_*.sh, scripts/q3_openarena_compat_check.sh, regression/smoke scripts.

Knowledge gaps addressed

• When tile cull runs vs opaque depth (r_forwardPlusDepthCull).
• How authored VDB density blends into froxel fog and which cvars gate it.
• What the optional RTX entity BLAS adds on top of the world BSP demo path.
• How TRELLIS mirrors FLUX operationally and where main-thread RegisterModel applies.
• Where Q3/OpenArena and safe-default invariants are enforced in CI-style scripts.

Testing

Documentation-only follow-up commit verified against src/renderers/vulkan/tr_init.c and scripts/q3_openarena_compat_check.sh.