Adds code comments documenting the WebGPU transient ("memoryless") attachment validation invariants that the engine already satisfies, so future edits don't accidentally break them.

WebGPU 149/150 tightened validation for textures created with the TRANSIENT_ATTACHMENT usage flag. The engine's transient color/depth attachments already comply, but the constraints were implicit. The comments record them at the two texture descriptors in WebgpuRenderTarget:

Changes:

• Document the three transient-attachment invariants next to the transient depth and color descriptors in webgpu-render-target.js:
  • the texture descriptor must not set viewFormats (must be an empty array);
  • the texture view must not override usage;
  • a transient texture must only ever be a render view, never a resolveTarget.

No functional change — comments only.