Add doctest coverage for FragmentId Display and AsRef<str> trait implementations

[coderabbitai]

Summary

Two public trait implementations in crates/whitaker_clones_core/src/index/fragment_id.rs introduced in PR #218 have no test coverage of any kind:

Item	Status
impl fmt::Display for FragmentId	No unit test, no doctest
impl AsRef<str> for FragmentId	No unit test, no doctest

Both are single-line delegations to as_str, so the risk is low, but coverage should be present to meet the project standard of not treating warnings as optional.

Proposed resolution

Add one inline /// # Examples doctest block to each implementation's declaration site in crates/whitaker_clones_core/src/index/fragment_id.rs:

1. Display doctest — assert that format!("{id}") produces the same string as the underlying value.
2. AsRef<str> doctest — assert that .as_ref() returns the same slice as .as_str().

References

• PR: Bridge lemma for FragmentId ordering; Verus proof for CandidatePair::new (7.2.6) #218
• Comment: Bridge lemma for FragmentId ordering; Verus proof for CandidatePair::new (7.2.6) #218 (comment) (raised by @leynos)

[leynos]

Agent prompt:

File: crates/whitaker_clones_core/src/index/fragment_id.rs

Two public trait implementations are missing test coverage:
1. `impl fmt::Display for FragmentId`
2. `impl AsRef<str> for FragmentId`

Task: Add one inline `/// # Examples` doctest block to each implementation's
declaration site. Each doctest must:

1. `Display` doctest — assert that `format!("{id}")` produces the same string as
   the underlying value, e.g.:
   ```rust
   use whitaker_clones_core::FragmentId;
   let id = FragmentId::new("src/lib.rs:10..20");
   assert_eq!(format!("{id}"), "src/lib.rs:10..20");
   ```

2. `AsRef<str>` doctest — assert that `.as_ref()` returns the same slice as
   `.as_str()`, e.g.:
   ```rust
   use whitaker_clones_core::FragmentId;
   let id = FragmentId::new("src/lib.rs:10..20");
   let slice: &str = id.as_ref();
   assert_eq!(slice, "src/lib.rs:10..20");
   ```

Do not modify any other part of the file. Do not alter existing doc comments or
existing doctests.