Specialize ASCII identifier lexing (#29503)
## Summary
We give ASCII identifiers a dedicated path, avoiding Unicode bookkeeping
and checking string prefixes only when a one- or two-letter name reaches
a quote. Both paths share the existing batched ASCII scanner, preserving
fast scanning for Unicode names with long ASCII suffixes.
Stacked on #29463 to measure the incremental improvement. [CodSpeed
reports](https://github.com/astral-sh/ruff/pull/29503/checks?check_run_id=118300735426)
6.6–9.8% greater efficiency across all five lexer benchmarks, with no
reported regressions. We preserve the existing scanner and add no
inlining attributes or unsafe code.
