Remove stale Vulkan compile-time feature flags (USE_VBO_GRID, USE_TESS_NEEDS, USE_VK_PBR #else)

[cursor]

Summary

Removes stale compile-time feature flags in renderers/vulkan/ that are permanently enabled and whose disabled branches are dead code. Preserves runtime toggles (vk.pbrActive, r_pbr, etc.).

Flags removed

Flag	Why safe to delete
USE_VBO_GRID	Always defined when USE_VBO is on (always). Grid VBO path is the only implementation; #ifdef guards were no-ops.
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2	Macros were commented out in tr_local.h; guards always evaluated false, so tess needsNormal/needsST2 plumbing was dead.
VERTEXSIZE 8 #else	USE_VK_PBR is unconditionally defined; PBR vertex layout (11 floats) is the only path.
USE_VK_PBR #else in vk_shader_modules.c / vk_shutdown.c	Non-PBR shader init/teardown paths unreachable; Vulkan renderer is PBR-only at compile time.
USE_VK_PBR header nesting	Flattened always-on VK_PBR_BRDFLUT / VK_CUBEMAP defines in tr_local.h.
USE_VK_PBR guard in vk_pbr_ibl_validate.c	Compile-time guard removed; runtime vk.pbrActive check retained.

Files touched

• renderers/vulkan/tr_local.h, tr_surface.c, tr_shade.c, tr_main.c, tr_animation.c
• renderers/vulkan/vk_shader_modules.c, vk_shutdown.c, vk_vbo.c
• renderers/vulkan/vk_pbr_ibl_validate.c

Behavioral parity

• Active paths unchanged: PBR vertex layout, grid VBO tessellation, generated PBR shaders, PBR IBL validation when vk.pbrActive.
• Runtime cvars (r_pbr, r_forwardPlus, etc.) untouched.

Candidates not removed (low confidence)

• Broader #ifdef USE_VK_PBR unwrapping across vk_*.c / tr_*.c (large diff, struct members still gated)
• Legacy ignored cvars r_allowSoftwareGL / r_glDriver (kept for mod cfg compatibility)
• Build-profile flags (USE_VULKAN_RTX, USE_EXPERIMENTAL_RENDERERS, etc.)

Test plan

• ./tests/scripts/test_vulkan_regression_source_guards.sh — PASS
• ./scripts/compile_engine.sh vulkan core — PASS
• ./scripts/smoke_test.sh ./build-vk-Release (headless VM: server/binary checks only)
• Manual client with mod data (not run in cloud agent)
• Manifest updated — N/A (no CMake gate changes)
• ./scripts/ci/audit_unconditional_sources.sh — N/A