grammar: add @"..." token-aware literal syntax
Add LLAMA_GRETYPE_TOKEN_LITERAL for token-aware grammar matching:
- Matches by token ID when literal resolves to single token
- Falls back to character-by-character matching otherwise
Also adds until_token() to PEG parser and updates to_gbnf() to emit
@"..." syntax for token() primitives, enabling token-aware constrained
decoding in generated grammars.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
