Claude review: rust: sync: Add SetOnce::reset()

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

Patch Review

**Verdict: Looks good.**

The `drop_val()` helper is a clean refactoring that avoids duplicating the drop logic between `reset()` and `Drop`.

**The safety contract on `drop_val()` is precise:**
```rust
/// # Safety
///
/// If this function returns `true`, `self` must be freed or `init` must be reset to `0`.
```

This correctly documents the obligation on the caller. Both callsites uphold it:
- `reset()` sets `init` to 0 if `drop_val()` returned true
- `Drop` doesn't need to reset init since the whole struct is being freed

**The invariant update is correct:**
```rust
-/// - `init` may only increase in value.
+/// - `init` may only increase in value, unless modified through a mutable reference.
```

This carves out the exact exception needed for `reset()`, which requires `&mut self`.

**The doctest is a good addition** demonstrating the populate/reset/re-populate cycle.

---

---
Generated by Claude Code Patch Reviewer