Fixed WebGL2 texture units for the clustered light culling grid/index textures -- plain
exported constants rather than renderer instance/class state, since the numbers are
shared by every WebGL2Renderer instance (see docs/adr/0007-clustered-lighting-webgl2-webgpu-only.md).
Texture units 8-11 are reserved for spot shadows, unit 12 for directional shadows (and dummy fallback),
unit 13 for WebGL2Renderer._RAW_DEPTH_UNIT (PCSS raw depth blocker-search), and units 14-15
for the cluster grid and cluster indices textures.
This keeps the entire reserved global texture unit range strictly at 8-15 (8 units total),
perfectly fitting within the WebGL2 specification's guaranteed minimum MAX_TEXTURE_IMAGE_UNITS
of 16 (units 0-15), leaving units 0-7 available for material textures without unit overflow.
Fixed WebGL2 texture units for the clustered light culling grid/index textures -- plain exported constants rather than renderer instance/class state, since the numbers are shared by every
WebGL2Rendererinstance (see docs/adr/0007-clustered-lighting-webgl2-webgpu-only.md). Texture units 8-11 are reserved for spot shadows, unit 12 for directional shadows (and dummy fallback), unit 13 forWebGL2Renderer._RAW_DEPTH_UNIT(PCSS raw depth blocker-search), and units 14-15 for the cluster grid and cluster indices textures. This keeps the entire reserved global texture unit range strictly at 8-15 (8 units total), perfectly fitting within the WebGL2 specification's guaranteed minimumMAX_TEXTURE_IMAGE_UNITSof 16 (units 0-15), leaving units 0-7 available for material textures without unit overflow.