Add Queue::submit() for command buffer submission

[MarijnS95]

Depends on #1033

Move command buffer submission logic from each backend's Device into Queue::submit(), which takes ownership of the command buffers. Each backend uses its Fence abstraction (#1007) for GPU synchronization:

• Metal: commit() + waitUntilCompleted()
• Vulkan: vkQueueSubmit() signaling a timeline semaphore (VulkanFence), then VulkanFence::waitForCompletion()
• DX12: ExecuteCommandLists() + Queue::Signal() on the queue-owned DXFence, then DXFence::waitForCompletion()

Co-Authored-By: Claude Opus 4.6 (1M context) [email protected]

[MarijnS95]

This PR is in a bit of an odd interim state. It aims to at least generalize the submit+wait paths behind the common CommandBuffer abstraction, but there are still two points of contention:

1. Each backends' InvocationState still stores a "downcast" CommandBuffer. Storing the "trait" offloadtest::CommandBuffer requires extra casting in many places, or a casting helper that Claude generated here: Traverse-Research@8ddb409.
2. This function is subject to change pending future plans. Right now it still waits for completion, but in the near future this function should default to submitting asynchronously and instead returning the relevant fence and "to-be-signalled value".
   That however also relies on a keepalive mechanism and less strictly defined resource ownership and destruction, something that was brought up for debate previously and likely requires some more thought and confirmation before committing to that pattern.

[bogner]

This could take a llvm::SmallVectorImpl<std::unique_ptr<CommandBuffer>> &&, which would allow it to be called regardless of the actual size of the caller's SmallVector.

[bogner]

If we do the above, then this SmallVector can have a fixed size of 1, rather than using a bunch of extra stack space here.

Suggested change

	llvm::SmallVector<std::unique_ptr<CommandBuffer>> CBs;
	CBs.push_back(std::move(CB));
	return submit(std::move(CBs));
	return submit(
	llvm::SmallVector<std::unique_ptr<CommandBuffer>, 1>{std::move(CB)});