fix: silence clippy::map_unwrap_or in rules_overhead (unblocks Clippy CI)

[cedric013]

Summary

Closes #768.

cargo clippy --all-features -- -D warnings (the Clippy job) currently fails on main:

error: called `map(<f>).unwrap_or_else(<g>)` on a `Result` value
   --> rust/src/core/rules_overhead.rs:143:25
    = note: `-D clippy::map-unwrap-or` implied by `-D warnings`

Introduced by bf4ee7bf (symlink dedup, #759). This also fails the Embed SDK (lean-ctx-sdk) job, which clippy-checks the engine lib as a dependency.

Fix: use map_or_else (clippy's own suggestion). Behaviour is unchanged — canonicalize the path to a string, else fall back to the original f.path.

let canonical = std::fs::canonicalize(&f.path)
    .map_or_else(|_| f.path.clone(), |p| p.to_string_lossy().to_string());

Test plan

• cargo clippy --all-features -- -D warnings -> passes (no errors)
• cargo fmt --check

Notes for reviewers

• Risk areas / edge cases: none — pure lint fix, identical control flow (Ok -> stringified path, Err -> original path).
• Backwards compatibility: unaffected.

[yvgude]

Closing — the identical rules_overhead.rs Clippy fix already landed via PR #765 (commit f32c558). #768 is resolved.