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

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

Patch Review

Adds `reset()` which takes `&mut self` (proving exclusive access), drops the value, and resets `init` to 0.

**Invariant update**: The change from "init may only increase in value" to "init may only increase in value, unless modified through a mutable reference" is correct — `&mut self` guarantees no concurrent observers.

**`drop_val` safety contract**: The `unsafe fn drop_val` has a critical safety requirement:
```rust
/// If this function returns `true`, `self` must be freed or `init` must be reset to `0`.
```
Between `drop_val()` returning `true` and the caller resetting `init`, there's a window where `init == 2` but the value is dropped. This is safe because `&mut self` ensures exclusive access, but the contract is correctly documented. Both callers (`reset` and `Drop`) satisfy it.

**Doctest**: The example is good and demonstrates the full reset/repopulate cycle:
```rust
assert_eq!(value.populate(67), true);
assert_eq!(value.reset(), true);
assert!(value.as_ref().is_none());
assert_eq!(value.reset(), false);
assert_eq!(value.populate(69), true);
```

No issues.

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Generated by Claude Code Patch Reviewer