Claude review: gpu: nova-core: gsp: add mechanism to wait for space on command queue

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This replaces the immediate `EAGAIN` return with a `read_poll_timeout` loop, which is the right approach for supporting continuation records that can fill the queue.

The `driver_write_area_size` method is a good addition that cleanly computes the available space without needing to materialize the slices. The comment explaining the minimum value is helpful.

One observation: the `read_poll_timeout` uses `Delta::ZERO` for the poll interval:

```rust
read_poll_timeout(
    || Ok(self.driver_write_area_size()),
    |available_bytes| *available_bytes >= size_of::<GspMsgElement>() + size,
    Delta::ZERO,
    timeout,
)?;
```

A zero poll interval means busy-waiting. Other callers in the same tree (e.g., `wait_for_msg` at cmdq.rs:572) use `Delta::from_millis(1)`. This could be intentional since command queue space should free up very quickly after GSP processes a command, but it might be worth adding a brief comment explaining the choice or using a small interval to avoid CPU spinning under load.

The timeout of 1 second (`ALLOCATE_TIMEOUT`) matches nouveau's approach, which is good.

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Generated by Claude Code Patch Reviewer