perf(swc_ts_fast_strip): add ASCII fast path for strip-only replacement loop

[kdy1]

Summary

The strip-only replacement pass decodes replacement ranges as UTF-8 chars one-by-one, with no ASCII fast path.

Evidence

• Replacement loop: crates/swc_ts_fast_strip/src/lib.rs:342-387.
• For every replacement span, it iterates char_indices() and runs a large whitespace/newline match for each character.

Why this is a performance risk

Most TypeScript source is ASCII-heavy. Full UTF-8 char decoding and large match branching on every character can dominate strip-only runtime for heavily annotated files.

Suggested direction

• Add an ASCII fast path (byte-wise) for spans that are ASCII.
• Keep the current Unicode-safe slow path for non-ASCII spans.
• Consider normalizing/merging adjacent replacement ranges before mutation to reduce loop overhead.

Benchmark idea

Profile strip-only runs on large ASCII-dominant TS files (and mixed Unicode fixtures) to verify speedup without semantic regressions.