Improve grammar parser error recovery around malformed productions

[ehwan]

Summary

• Add targeted error recovery productions for malformed rule bodies, symbol bindings, terminal sets, reduce actions, and %allow targets.
• Propagate precedence declarations for the reserved error terminal into the core parser builder.
• Document that error can participate in precedence declarations for recovery-related conflict resolution.
• Add parser tests covering the new recovery paths and regenerate the bootstrapped parser output.

[gemini-code-assist]

Code Review

This pull request adds support for using the reserved error terminal in precedence declarations to resolve shift/reduce conflicts from panic-mode recovery productions. It updates the parser rules to handle malformed inputs gracefully, adds corresponding unit tests, and updates the documentation. Feedback is provided regarding a typo (fisrt instead of first) in the parser error recovery logic.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.