peg-parser: add preserved() for token-aware matching
Add common_peg_preserved_parser type that matches preserved tokens
(like <tool_call>, <escape>) at the token level when token info is
available, with graceful fallback to text matching.
New features:
- common_peg_token_span struct for token boundary info
- common_peg_parse_context extended with optional token spans
- preserved(literal, token_id) builder method
- Token-aware matching: if token_id set and context has tokens,
match by token ID; otherwise fall back to text
This is the infrastructure for token-aware grammars as described in
docs/development/token-aware-grammars.md.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
