feat: implement TypedAST construction in GrammarInterpreter

Add runtime TypedAST construction to GrammarInterpreter, enabling direct
parsing and AST generation without code generation. This allows embedded
DSL use cases and rapid prototyping with the ZynPEG 2.0 grammar system.

Changes:
- Add action executor to GrammarInterpreter that evaluates ActionIR and
  constructs TypedAST nodes (TypedStatement, TypedExpression, etc.)
- Add Float variant to ParsedValue for floating-point literals
- Add TypedAST variants to ParsedValue (Statement, Expression, Declaration,
  Program, Block) for returning constructed AST nodes
- Implement helper function execution (intern, parse_int, parse_float, text)
- Handle Box::new() as transparent wrapper in action evaluation
- Fix repeat binding accumulation - inner bindings now collect into lists
- Fix text() helper to use __text__ binding from atomic rules
- Add comprehensive tests demonstrating TypedAST construction from
  vintage.imgpipe parsing

Author: darmie