Claude review: rust: auxiliary: add registration_data_with() for ForLt types

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

Patch Review

The private helper extraction is clean:

```rust
unsafe fn registration_data_pinned<F: ForLt + 'static>(&self) -> Result<Pin<&F::Of<'static>>>
```

The `registration_data_with` closure-based API correctly uses an HRTB closure:

```rust
pub fn registration_data_with<F: ForLt + 'static, R>(
    &self,
    f: impl for<'a> FnOnce(Pin<&'a F::Of<'a>>) -> R,
) -> Result<R>
```

The soundness argument is correct: the `for<'a>` quantification prevents the closure from capturing external references with a concrete short lifetime and smuggling them into the invariant type. The round-trip (`'bound` → `'static` → `'a`) is safe because `'a` is universally quantified.

Relaxing `Registration`'s bound from `CovariantForLt` to `ForLt` is the right move — it lets invariant types be registered while `registration_data()` (requiring `CovariantForLt`) remains available as a convenience.

**Minor observation:** The three separate `unsafe` blocks in `registration_data_with` could arguably share a single safety comment since they're all justified by the same HRTB argument. The current form with individual comments is verbose but not wrong.

---
Generated by Claude Code Patch Reviewer