Feature-flag cleanup: remove stale Vulkan compile-time flags

[cursor]

Summary

Removes stale compile-time feature flags and dead non-PBR fallback branches in the Vulkan renderer. USE_VK_PBR is unconditionally defined in tr_local.h; the removed #else paths could not be built and only added maintenance burden.

Branch: cursor/feature-flag-cleanup-e683 → main

Flags removed / unwrapped

Flag / guard	Why safe
USE_VK_PBR #else in vk_shader_modules.c, vk_shutdown.c	Non-PBR shader module init and teardown paths; Vulkan shipping build always defines USE_VK_PBR
USE_VBO_GRID	Always enabled when USE_VBO is on; grid surfaces always use VBO path
USE_TESS_NEEDS_NORMAL / USE_TESS_NEEDS_ST2	Macros were commented out (disabled); dead tess attribute plumbing removed
VERTEXSIZE 8 #else	PBR vertex layout (11 floats) is the only supported layout
USE_VK_PBR header nesting	Flattened always-on VK_PBR_BRDFLUT / VK_CUBEMAP defines
USE_VK_PBR #else in vk_draw_state.c, vk_create_pipeline.c, vk.h, vk_attachments.c, vk_forward_plus.c, tr_material_paint.c	Dead non-PBR vertex binding counts (8 vs 10), descriptor set count (5 vs 20), shader module table sizes, pipeline factory switch, and material-paint color offsets (index 7 vs 10)
vk_pbr_ibl_validate.c compile guard	File only meaningful when PBR is compiled; runtime vk.pbrActive / r_pbr gating preserved

Behavioral parity

• Active PBR/VBO code paths unchanged; only unreachable #else branches and guards removed.
• Runtime toggles preserved: r_pbr, vk.pbrActive, Forward+, deferred, material blend, etc.

Test plan

• ./tests/scripts/test_vulkan_regression_source_guards.sh
• ./scripts/compile_engine.sh vulkan core
• ./scripts/smoke_test.sh ./build-vk-Release (headless VM; not run this iteration)
• Manual client with game data

Remaining candidates (not changed — low confidence)

• One-sided #ifdef USE_VK_PBR guards in tr_local.h / tr_shader.c (struct fields; no #else)
• Legacy cvars: r_allowSoftwareGL, r_glDriver
• Runtime feature cvars: r_forwardPlus, r_renderMode, SKIP_IDPAK_CHECK