feat: GPU optimization — lighting, viewport CA, benchmark mode, 531 FPS

- Resolution: 8x8 world cells, 2x2 UI cells (UI_SCALE=4)
- GPU lighting: vertex-shader computed, light source list buffer (max 64)
  instead of O(N×R²) grid scan, O(N×S) per cell
- Viewport-aware CA: iterate only active chunks, not all 250×250
- Flat array entity/item/shadow maps instead of HashMaps
- Flat 128-entry ASCII atlas array instead of HashMap lookup
- Partial grid upload: viewport + 30-cell margin only
- Pre-allocated viewport arrays in renderer structs (zero alloc/frame)
- Skip CPU lighting for GPU modes (pass None)
- Benchmark mode: --mode benchmark with per-subsystem timing
- GpuLightSource struct, light_count in push constants
- gather_sources_in_range() for viewport-scoped source gathering

Benchmark (600 ticks, release):
  Graphics: 531 FPS (was 386, +38%), render 1013us (was 1699us, -40%)
  ASCII:    402 FPS (was 313, +28%), render 1502us (was 2346us, -36%)

All 171 tests + 14 scenarios pass.

Cargo.toml

@@ -14,6 +14,7 @@ ash = "0.38"
 ash-window = "0.13"
 raw-window-handle = "0.6"
 bytemuck = { version = "1", features = ["derive"] }
+image = { version = "0.25", default-features = false, features = ["png"] }
 
 [profile.release]
 opt-level = 3