Claude review: gpu: nova-core: send UNLOADING_GUEST_DRIVER GSP command upon unloading

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

Patch Review

This is the key functional patch. The design is clean: `driver::unbind` -> `gpu::unbind` -> `gsp::unload` -> `shutdown_gsp`.

The `PowerStateLevel` enum with `#[expect(unused)]` is appropriate for Level3/Level7 since only Level0 (full unload) is used currently:

```rust
+#[expect(unused)]
+pub(crate) enum PowerStateLevel {
+    Level0 = bindings::NV2080_CTRL_GPU_SET_POWER_STATE_GPU_LEVEL_0,
+    Level3 = bindings::NV2080_CTRL_GPU_SET_POWER_STATE_GPU_LEVEL_3,
+    Level7 = bindings::NV2080_CTRL_GPU_SET_POWER_STATE_GPU_LEVEL_7,
+}
```

The `shutdown_gsp` function polls for the LIBOS_INTERRUPT_PROCESSOR_SUSPENDED bit:

```rust
+const LIBOS_INTERRUPT_PROCESSOR_SUSPENDED: u32 = bits::bit_u32(31);
+read_poll_timeout(
+    || Ok(gsp_falcon.read_mailbox0(bar)),
+    |&mb0| mb0 & LIBOS_INTERRUPT_PROCESSOR_SUSPENDED != 0,
+    Delta::from_millis(10),
+    Delta::from_secs(5),
+)
```

Minor: `LIBOS_INTERRUPT_PROCESSOR_SUSPENDED` is defined as a local const. For consistency with other register constants in this driver, consider whether this belongs in `regs.rs` or in the bindings, though the current placement is fine for a single use site.

The `UnloadingGuestDriverReply` reads `type Message = ()` and the `read()` method ignores its arguments, which is correct for a command that has no meaningful reply payload.

The `unsafe impl AsBytes` and `unsafe impl FromBytes` on `UnloadingGuestDriver` are justified by the SAFETY comments referencing the struct's contents (integer types + explicit padding).

No blocking issues.

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Generated by Claude Code Patch Reviewer