Improve Latin word-boundary replacement to cut partial-word hits

[fishonbike]

Surface replacement is longest-first literal substring. For Latin text this can hit inside longer words. Add word-boundary guards for ASCII surfaces (CJK unaffected) to improve precision, with tests.

[Diyaaa-12]

Hi @fishonbike, I'd like to work on this issue. I'll add word-boundary guards for ASCII surfaces in the Latin replacement logic and include tests to verify precision improvements. Could you please assign this to me? Thank you!