Migrate GIB parser to petitparser

[vijaytogra]

• Refactored GIB parser to use petitparser library (matching SGF parser)
• Replaced hand-written imperative parser with declarative grammar
• Removed obsolete parse_ext.dart file that was only used by GIB parser
• Maintains same public API and data structures for backward compatibility
• All 15 tests pass without modification

[vijaytogra]

Summary

Migrated the GIB parser from a hand-written imperative parser to use the petitparser library, bringing consistency with the SGF parser implementation.

Changes

• ✅ Refactored lib/parse/gib/gib.dart to use petitparser GrammarDefinition
• ✅ Removed lib/parse/parse_ext.dart (no longer needed)
• ✅ Headers section: Full declarative parser with support for key:value pairs and plain values
• ✅ Game section: Simplified pattern-based parsing for robustness
• ✅ Maintains same public API - no breaking changes
• ✅ All 15 tests pass without modification

Benefits

• Consistency: Both parsers (SGF & GIB) now use the same parsing library
• Maintainability: Declarative grammar is easier to understand and modify
• Reduced Code: Removed custom extension methods, cleaner codebase

Testing

All existing tests pass:

flutter test test/gib_test.dart  # ✅ Pass
flutter test                      # ✅ All 15 tests pass

[benjaminpjones]

Hi Vijay, I'm not as familiar with the GIB side, but I want to note that the GIB test suite is not very robust. There is only one test in gib_test.dart, and the assertions are pretty loose.

If you want to be confident that this doesn't cause a regression, I recommend adding a test that exercises GameRecord.fromGib and asserts the moves that are read out.