feat(rendering)!: blend each attachment of a multi-target draw on its own terms

[Exoridus]

One blend mode per colour attachment of a multi-target draw. Until now a single blend setting applied to the whole draw; a G-buffer wants alpha blending on albedo and none on a normal or id slot, where a blended vector or id is meaningless.

MeshMaterial takes blendModes, one entry per attachment index in fragment-output order; entries past the target's attachment count are ignored, attachments past the end of the list keep the draw's own mode, and a list entry wins over a per-mesh blend override on the attachments it covers. The list is part of the pipeline key. On WebGPU each targets[i].blend is set from it, one code path for single- and multi-attachment targets. On WebGL2 it needs OES_draw_buffers_indexed: when the effective modes agree the whole-draw call is issued and no extension is needed; when they differ the indexed calls are issued per attachment and restored afterwards by re-issuing the non-indexed state, which resets every draw buffer. Differing modes on a device without the extension throw a RenderError before the draw is queued, from the same entry point that already refuses a material that does not write every attachment, so nothing is pooled and no transform slot is consumed. The capability is RenderBackend.supportsPerAttachmentBlend, re-probed after context loss. An instanced batch refuses a multi-attachment target on both backends the same way, because the batch pipeline is built for one target.

Tests: unit coverage of plumbing, pipeline-key identity, the indexed GL calls and their restore (including the cached-mode trap where the restore would otherwise be suppressed), the refusal through renderTo with a clean following frame; browser readback in both lanes with a two-attachment target and [Normal, Additive], identical numbers on WebGL2 and WebGPU. Marked breaking because RenderBackend gains a member a third-party backend has to implement.

[codecov]

Bundle Report

Changes will increase total bundle size by 35.1kB (0.11%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
exo-esm-esm	1.27MB	2.93kB (0.23%) ⬆️
exo-esm-modules-esm	3.66MB	8.96kB (0.25%) ⬆️
exo-iife-min-Exo-iife	1.27MB	2.92kB (0.23%) ⬆️
exo-full-iife-Exo-iife	5.49MB	8.75kB (0.16%) ⬆️
exo-iife-Exo-iife	3.54MB	8.61kB (0.24%) ⬆️
exo-full-iife-min-Exo-iife	1.99MB	2.94kB (0.15%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: exo-iife-Exo-iife

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
exo.iife.js	8.61kB	3.54MB	0.24%

Files in exo.iife.js:

• ./src/rendering/RenderBackend.ts → Total Size: 624 bytes

• ./src/rendering/webgpu/WebGpuMeshRenderer.ts → Total Size: 76.85kB

• ./src/rendering/webgl2/WebGl2Backend.ts → Total Size: 86.69kB

• ./src/rendering/multiAttachmentGuard.ts → Total Size: 5.65kB

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.89kB

• ./src/rendering/material/MeshMaterial.ts → Total Size: 3.88kB

• ./src/rendering/webgl2/WebGl2MeshRenderer.ts → Total Size: 37.24kB

• ./src/rendering/webgpu/WebGpuBackend.ts → Total Size: 95.75kB

view changes for bundle: exo-full-iife-min-Exo-iife

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
exo.full.iife.min.js	2.94kB	1.99MB	0.15%

Files in exo.full.iife.min.js:

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.89kB

view changes for bundle: exo-full-iife-Exo-iife

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
exo.full.iife.js	8.75kB	5.49MB	0.16%

Files in exo.full.iife.js:

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.89kB

view changes for bundle: exo-esm-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
exo.esm.js	2.93kB	1.27MB	0.23%

Files in exo.esm.js:

• ./src/rendering/material/MeshMaterial.ts → Total Size: 3.8kB

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.69kB

• ./src/rendering/RenderBackend.ts → Total Size: 615 bytes

• ./src/rendering/multiAttachmentGuard.ts → Total Size: 5.55kB

• ./src/rendering/webgl2/WebGl2Backend.ts → Total Size: 84.58kB

• ./src/rendering/webgpu/WebGpuBackend.ts → Total Size: 93.42kB

• ./src/rendering/webgl2/WebGl2MeshRenderer.ts → Total Size: 36.47kB

• ./src/rendering/webgpu/WebGpuMeshRenderer.ts → Total Size: 75.08kB

view changes for bundle: exo-iife-min-Exo-iife

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
exo.iife.min.js	2.92kB	1.27MB	0.23%

Files in exo.iife.min.js:

• ./src/rendering/webgpu/WebGpuBackend.ts → Total Size: 95.75kB

• ./src/rendering/multiAttachmentGuard.ts → Total Size: 5.65kB

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.89kB

• ./src/rendering/RenderBackend.ts → Total Size: 624 bytes

• ./src/rendering/webgl2/WebGl2MeshRenderer.ts → Total Size: 37.24kB

• ./src/rendering/material/MeshMaterial.ts → Total Size: 3.88kB

• ./src/rendering/webgpu/WebGpuMeshRenderer.ts → Total Size: 76.85kB

• ./src/rendering/webgl2/WebGl2Backend.ts → Total Size: 86.69kB

view changes for bundle: exo-esm-modules-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
rendering/webgpu/WebGpuBackend.js	213 bytes	96.25kB	0.22%
rendering/webgl2/WebGl2Backend.js	3.7kB	87.21kB	4.43%
rendering/webgpu/WebGpuMeshRenderer.js	150 bytes	76.31kB	0.2%
rendering/webgl2/WebGl2MeshRenderer.js	377 bytes	37.27kB	1.02%
rendering/material/MaterialKey.js	139 bytes	9.0kB	1.57%
rendering/multiAttachmentGuard.js	2.47kB	6.04kB	69.21% ⚠️
rendering/material/MeshMaterial.js	1.91kB	4.13kB	85.43% ⚠️

Files in rendering/webgpu/WebGpuBackend.js:

• ./src/rendering/webgpu/WebGpuBackend.ts → Total Size: 93.42kB

Files in rendering/webgl2/WebGl2Backend.js:

• ./src/rendering/webgl2/WebGl2Backend.ts → Total Size: 84.58kB

Files in rendering/webgpu/WebGpuMeshRenderer.js:

• ./src/rendering/webgpu/WebGpuMeshRenderer.ts → Total Size: 75.03kB

Files in rendering/webgl2/WebGl2MeshRenderer.js:

• ./src/rendering/webgl2/WebGl2MeshRenderer.ts → Total Size: 36.4kB

Files in rendering/material/MaterialKey.js:

• ./src/rendering/material/MaterialKey.ts → Total Size: 8.68kB

Files in rendering/multiAttachmentGuard.js:

• ./src/rendering/multiAttachmentGuard.ts → Total Size: 5.55kB

Files in rendering/material/MeshMaterial.js:

• ./src/rendering/material/MeshMaterial.ts → Total Size: 3.8kB