issue_body.txt

The `as_str` methods on `Severity`, `Scope`, and `FailOn` in `crates/diffguard-types/src/lib.rs` (lines 52, 71, 90) are flagged by `clippy::missing_const_for_fn`:

```rust
pub fn as_str(self) -> &'static str {
    match self {
        Severity::Info => "info",
        Severity::Warn => "warn",
        Severity::Error => "error",
    }
}
```

These should be `pub const fn as_str` — the match is a simple constant expression with no side effects, and making them const enables compile-time evaluation. Low effort fix, just add `const` to all three.

Note: `#185` already covers adding `#[must_use]` to these same methods — the two issues could theoretically be combined, but they are distinct fixes so keeping separate.