Claude review: rust/drm/gem: Use DeviceContext with GEM objects

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

Patch Review

**GAT (Generic Associated Type) usage:**
```rust
type Object<Ctx: drm::DeviceContext>: AllocImpl;
```
This is a significant API change, making `Object` a GAT. This is well-motivated — it allows GEM objects to carry their device's context state.

**VTABLE initialization with arbitrary `Uninit` context:**
```rust
// Ignore the Uninit DeviceContext below. It is only provided because it is required by the
// compiler, and it is not actually used by these functions.
gem_create_object: T::Object::<Uninit>::ALLOC_OPS.gem_create_object,
```
The comment explains this well. Since `ALLOC_OPS` is a `const` that doesn't depend on the context type, the choice of `Uninit` is arbitrary and correct. It would be slightly cleaner if `AllocOps` were associated with the driver rather than the object type, but that would be a larger refactor.

**`open_callback` / `close_callback` use `Registered` context:**
```rust
let obj: &DriverAllocImpl<T> = unsafe { IntoGEMObject::from_raw(raw_obj) };
```
Since `DriverAllocImpl<T>` defaults to `DriverAllocImpl<T, Registered>`, this correctly uses the `Registered` context for callbacks that can only be invoked after registration. The safety comment explicitly calls this out — good.

**`Object::dev()` propagates `Ctx`:**
```rust
pub fn dev(&self) -> &drm::Device<T::Driver, Ctx> {
```
This is correct — if you have an `Object<T, Uninit>`, you get back a `&Device<T, Uninit>`, which appropriately restricts what you can do with the device.

**`BaseObject::create_handle` and `lookup_handle` bound changes:**
```rust
D: drm::Driver<Object<Registered> = Self, File = F>,
```
This correctly constrains these operations to only work with `Registered` context objects, since handle operations require a registered device.

**Overall:** This patch ties the series together cleanly. The PhantomData addition to `Object` is zero-cost, and the context propagation through the type system is sound.

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