feat: implement ZynPEG 2.0 Phase 3 - TypedAST builder helpers

Add typed_ast module with helper functions for TypedAST construction:
- AstContext for managing builder and type registry
- Expression builders: int_literal, binary, call, method_call, etc.
- Statement builders: let_stmt, if_stmt, while_stmt, for_stmt, etc.
- Declaration builders: function_decl, variable_decl, class_decl
- Operator helpers: parse_binary_op, parse_unary_op
- Fold helpers: fold_binary_left, fold_binary_right
- ParsedValue conversion helpers for runtime integration

All field names and types match the actual zyntax_typed_ast structures.

Author: darmie

crates/zyn_peg/src/lib.rs

@@ -47,6 +47,7 @@ pub mod runtime;
 pub mod grammar;
 pub mod runtime2;
 pub mod codegen;
+pub mod typed_ast;
 
 // Parser for .zyn grammar files
 #[derive(Parser)]