Claude review: gpu: nova: separate driver type from driver data

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

Patch Review

This splits `NovaDriver` into a unit struct (for trait impls) and a new `Nova` struct (for driver data), enabling the `Data` GAT to be lifetime-parameterized in the future.

```rust
-pub(crate) struct NovaDriver {
+pub(crate) struct NovaDriver;
+
+pub(crate) struct Nova {
     #[expect(unused)]
-    drm: ARef<drm::Device<Self>>,
+    drm: ARef<drm::Device<NovaDriver>>,
 }
```

The `Self` in `ARef<drm::Device<Self>>` correctly becomes `NovaDriver` since the field is now on `Nova`, not `NovaDriver`.

```rust
-    type Data<'bound> = Self;
+    type Data<'bound> = Nova;
```

And the probe return type changes:

```rust
-    ) -> impl PinInit<Self, Error> + 'bound {
+    ) -> impl PinInit<Self::Data<'bound>, Error> + 'bound {
```

With:

```rust
-        Ok(Self { drm })
+        Ok(Nova { drm })
```

This is clean. Note the `Data` type uses a GAT with `'bound` which already appears in the base code (from the prerequisite series). The separation is a standard Rust pattern to allow the data type to carry lifetimes that the driver trait impl type cannot.

**Minor nit:** The existing `Convienence` typo in the doc comment (line 860/`NovaDevice` type alias) is pre-existing and not introduced by this patch.

No issues found.

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