[Web] Batch GPU-to-GPU copies, fix WebGPU synchronization, and add tests for command batching

[akaashrp]

Batch GPU-to-GPU buffer copies with pending compute dispatches in the same GPUCommandEncoder instead of finishing and submitting after every copy. Since copyBufferToBuffer records an encoder command, dependencies remain ordered while copies can be submitted at the next required synchronization boundary (see https://www.w3.org/TR/webgpu/#buffer-copies).

Preserve pending GPU-to-CPU readbacks separately from their queue-tail status so sync() waits for both host readback completion and any later queue work when necessary. Also flush pending commands before canvas drawing to prevent rendering stale buffer contents.

Add WebGPU device mocks and tests covering:

• Mixed compute and GPU-copy batching
• Host-write, readback, deallocation, and canvas-draw flush boundaries
• deviceCopyFromGPU followed by a batched GPU copy and sync()
• Readback error propagation

Regarding importing from ../../src/webgpu instead of ../../dist: WebGPUContext is not re-exported from index.ts, so it is unavailable through the built package like for the other tests. Exporting it solely for this test would unnecessarily expand the public API. Importing ../../src/webgpu is a relatively simple alternative.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[guan404ming]

Look nice, left some comments.

web/src/webgpu.ts:531 (outside the diff): could we flush in drawImageFromBuffer too? It submits its own encoder and may render stale data.

[akaashrp]

Thanks @guan404ming, should be addressed now

[guan404ming]

Looks nice, thanks!