Claude review: rust: page: convert to `Ownable`

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

Patch Review

This converts `Page` from a directly-owned type (with `Drop`) to an `Ownable` type (used via `Owned<Page>`).

Key change: `Page` becomes `#[repr(transparent)]` wrapping `Opaque<bindings::page>` instead of `NonNull<bindings::page>`. This allows the `Page` type to be used as a reference target (like `&Page`) rather than only as a move-value.

```rust
pub struct Page {
    page: Opaque<bindings::page>,
}
```

`alloc_page` now returns `Owned<Page>` instead of `Page`:
```rust
pub fn alloc_page(flags: Flags) -> Result<Owned<Self>, AllocError> {
```

The `BorrowedPage` internals change from `ManuallyDrop<Page>` to `ManuallyDrop<NonNull<Page>>`, which makes sense since `Page` is no longer directly constructible.

The `Ownable` impl correctly frees the page:
```rust
impl Ownable for Page {
    unsafe fn release(&mut self) {
        let ptr: *mut Self = self;
        unsafe { bindings::__free_pages(ptr.cast(), 0) };
    }
}
```

This works because `Page` is `#[repr(transparent)]` over `Opaque<bindings::page>`, so `ptr.cast()` gives a valid `*mut bindings::page`.

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Generated by Claude Code Patch Reviewer