Claude review: rust: module: add `THIS_MODULE` const to `ModuleMetadata` trait

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

Patch Review

This patch adds `const THIS_MODULE: ThisModule` to the `ModuleMetadata` trait, moves the `THIS_MODULE` static from the `module!` macro body into the `ModuleMetadata` impl for the module type, and updates `__init` to use the trait const.

The key change in `rust/kernel/lib.rs`:
```rust
const THIS_MODULE: ThisModule;
```

And in `rust/macros/module.rs`, the old `static THIS_MODULE` (with `#[cfg(MODULE)]` / `#[cfg(not(MODULE))]` variants) becomes:
```rust
impl ::kernel::ModuleMetadata for #type_ {
    const NAME: &'static ::kernel::str::CStr = #name_cstr;

    #[cfg(MODULE)]
    const THIS_MODULE: ::kernel::ThisModule = {
        extern "C" {
            static __this_module: ::kernel::types::Opaque<::kernel::bindings::module>;
        }
        unsafe { ::kernel::ThisModule::from_ptr(__this_module.get()) }
    };

    #[cfg(not(MODULE))]
    const THIS_MODULE: ::kernel::ThisModule = unsafe {
        ::kernel::ThisModule::from_ptr(::core::ptr::null_mut())
    };
}
```

**Observations:**

1. **`static` → `const` semantics for `THIS_MODULE`**: The old `static THIS_MODULE` had a stable address. Making it a `const` means each use site can potentially get its own copy. However, `ThisModule` is just a wrapper around `*mut module` — a pointer-sized value — so this should be fine. The pointer *value* (i.e., the address of `__this_module`) is the important thing, not the address of the `ThisModule` wrapper itself. Callers that need a `&ThisModule` reference (like `__init`) will take a reference to a temporary, which is valid in Rust for const promotion. This is correct.

2. The `SAFETY` comment is preserved and appropriate — `__this_module` is kernel-constructed and lives for the module lifetime. The `from_ptr(null_mut())` path for built-in is consistent with the previous behavior.

3. The `__init` call site change from `&super::super::THIS_MODULE` to `&<super::super::LocalModule as ::kernel::ModuleMetadata>::THIS_MODULE` is verbose but correct — the fully-qualified syntax avoids ambiguity.

**No issues found.**

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