Coalesce qualifying choices into character classes

Normalized profile

What this task asks for

Add optimizer passes that collapse qualifying choice chains into merged character and negated character classes. Add a CharClass(Vec<(String, String)>) variant and a NegCharClass(Vec<(String, String)>) variant to OptimizedExpr. Choice chains of qualifying alternatives collapse into CharClass holding merged character ranges. A RestoreOnErr-wrapped alternative qualifies when its inner expression qualifies. When only some qualify, contiguous runs of three or more qualifying alternatives are coalesced. A coalesced result is emitted only when merging produces fewer ranges than the original alternative count. A single merged range simplifies to Range when endpoints differ or Str when equal.