feat(lighting): Complete lighting overhaul — PCSS, SH L1 LPV, occlusion, LUT color grading

[MichaelFisher1997]

Summary

Completes the lighting system overhaul tracked in #143. This PR implements phases 4-6 and fixes the remaining Vulkan validation error.

Changes

• PCSS Soft Shadows (Phase 4): Replaced fixed-radius PCF with Percentage-Closer Soft Shadows using 8-sample Poisson disk blocker search and penumbra-based variable-radius filtering for contact-hardening shadows
• Native 3D Textures for LPV (Phase 5A): Migrated LPV grids from 2D atlas (image2D) to native VkImage3D / image3D, enabling hardware trilinear sampling
• Occlusion-Aware Propagation (Phase 5B): Added GPU SSBO occlusion grid built from world chunk data; opaque blocks now correctly block light propagation
• SH L1 Directional LPV (Phase 5C): Replaced scalar RGB LPV with Spherical Harmonics L1 encoding (3 channel textures × 4 SH coefficients each) for directionally-aware indirect lighting that responds to surface normals
• LUT Color Grading (Phase 6): Added 32³ 3D LUT-based color grading to the post-process pipeline with configurable intensity
• 3D Dummy Texture Fix: Created a 1×1×1 3D dummy texture so sampler3D descriptors (bindings 11-13) have a correctly-typed fallback before LPV initialization, fixing VK_IMAGE_VIEW_TYPE_2D vs Dim=3D validation errors

Files Changed (24 files)

Shaders: terrain.frag, lpv_inject.comp, lpv_propagate.comp, post_process.frag + SPIR-V binaries
Engine: lpv_system.zig, rhi.zig, rhi_vulkan.zig, rhi_types.zig, rhi_tests.zig, render_graph.zig
Vulkan backend: descriptor_manager.zig, descriptor_bindings.zig, rhi_context_types.zig, rhi_context_factory.zig, rhi_init_deinit.zig, rhi_frame_orchestration.zig, rhi_resource_setup.zig, rhi_shadow_bridge.zig, rhi_pass_orchestration.zig, post_process_system.zig
Game: world.zig

Testing

• zig build test passes (unit tests + SPIR-V shader validation)
• zig build compiles cleanly
• Pre-push hooks (formatting + full test suite) passed

Resolves #143

[MichaelFisher1997]

Closing in favor of a clean branch — the old branch had merge commits that caused phantom conflicts after #262 was squash-merged into dev.