Claude review: gpu: nova-core: gsp: clarify invariant on command queue

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Patch Review

Adds a comment explaining why only `dst.contents.0` is used for extracting the command header. This is a documentation-only change.

The comment:
```rust
// Extract area for the command itself. The GSP message header and the command header
// together are guaranteed to fit entirely into a single page, so it's ok to only look
// at `dst.contents.0` here.
```

This relies on the fact that `GspMsgElement` + any `Command` struct together fit in `GSP_PAGE_SIZE` (4096 bytes). This is a reasonable assumption for the fixed-size command headers currently in use, but worth noting this would break if a `Command` struct ever exceeded ~4KB. The invariant is implicitly enforced by the fact that the queue element starts page-aligned and `GspMsgElement::from_bytes_mut_prefix` splits at the element boundary.

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