Merge peg-simplify: enum-based tags and lambda mappers
Merge origin/peg-simplify branch which introduces:
- Enum-based AST tags (common_chat_peg_tag) instead of string tags
- Lambda-based mappers instead of class-based mappers
- Exhaustive switch statements for type-safe tag dispatch
Updated FunctionGemma parser to use the new style:
- Uses p.tag(Tag::XXX, ...) and p.atomic_tag() directly
- Removed specialized builder class
- Token-aware matching with p.token() for special tokens
Fixed constructed mapper to properly close string quotes in TOOL_CLOSE.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
