peg-parser: use enum-based tags instead of strings
Replace string-based AST tags with integer tag IDs backed by enums:
- Add common_chat_peg_tag enum with all chat parser tags
- Change common_peg_ast_node::tag (string) to tag_id (int)
- Add p.tag(enum, parser) and p.atomic_tag(enum, parser) template methods
- Refactor mappers to use switch statements for exhaustive dispatch
- Update preset.cpp with local ini_tag enum
Benefits:
- Type safety: compiler catches tag name typos
- Exhaustive switches: compiler warns about missing cases
- Faster dispatch: integer comparison vs string comparison
- Less boilerplate: direct enum usage instead of wrapper methods
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Co-Authored-By: Claude <noreply@anthropic.com>
