perf: avoid utf8 conversion when appending extensions

[hardfist]

Summary

Split out the first resolver micro-optimization from #215.

This keeps extension probing on Unix in OsStr byte form when appending configured extensions, avoiding a lossy UTF-8 string conversion on the normal file-extension lookup path.

Measurement

Measured previously against exact Callgrind for:

benches/resolver.rs::resolver::bench_resolver::resolver[single-thread]

Baseline: 116,917,041 Ir
After this change: 116,554,214 Ir
Delta: -362,827 Ir (-0.31%)

Artifact from the original measurement run:

optimization-artifacts/valgrind/callgrind/20260517T043726Z-single-thread-unix-extension-buffer/

Validation

cargo clippy --all-features -- -D warnings
cargo test --lib -- --skip tests::pnp

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing perf/unix-extension-buffer (d488231) with main (8975e38)}