feat(NFC): skip Unicode pass for all-ASCII inputs

[KimBioInfoStudio]

Summary

Adds a single-line ASCII gate to the NFC normalizer: when NormalizedString::get().is_ascii() is true the normalizer returns immediately, skipping the per-char unicode-normalization-alignments pass and the transform() rebuild it triggers.

Why this is safe

ASCII (U+0000..=U+007F) is NFC by Unicode invariant:

• no ASCII code point has a Decomposition_Mapping
• no ASCII code point combines with adjacent characters
• no ASCII code point is the target of any composition

Running the existing NFC code on an all-ASCII NormalizedString therefore rebuilds normalized and alignments to the exact same bytes and tuples it already had — the gate just elides that work.

The gate is strictly conservative: any non-ASCII byte in the input falls through to the original code path with zero changes. Combining-mark sequences (e.g. e + U+0301 → é), CJK, Arabic, Cyrillic, Vietnamese, Hindi, Thai, etc. all keep their existing behavior.

Tests

Two unit tests in normalizers::unicode::tests pin this:

• nfc_ascii_fast_path_is_no_op — runs NFKD on ff first to produce all-ASCII text with non-trivial alignments (each output byte still maps back to the 3-byte ligature), then asserts NFC.normalize leaves the entire NormalizedString byte-identical.
• nfc_non_ascii_still_runs_unicode_path — checks that e + combining acute is still composed to é.

Test plan

• cargo test --lib (202 tests pass)
• New equivalence + non-ASCII regression tests
• CI on x86_64 / Linux / macOS

Notes

This is a minimal-risk performance gate — no new dependencies, no unsafe, no SIMD. Companion to #2036 (SIMD ASCII fast path for Lowercase), but fully independent of it.

Same-shape gates may apply to NFD, NFKC, NFKD (ASCII is also already NFD/NFKD/NFKC by the same invariant); happy to extend in a follow-up if maintainers prefer. Kept minimal here for review clarity.

🤖 Generated with Claude Code

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.