Add function signature validation for type annotations

[nbeerbower]

Previously, function-type annotations (e.g., fn(i32): i32) only checked that a value was a function, without validating parameter counts, types, return types, or async flags. This change adds proper signature validation.

Changes:

• Add validate_function_signature() to types.c for comprehensive signature checking including parameter count, types, return type, and async flag
• Add types_compatible() helper for comparing types recursively
• Add format_type() helper for generating readable error messages
• Add type_copy() to ast.c for deep copying Type structures including all function-type fields (fn_param_types, fn_return_type, etc.)
• Update expressions.c to use type_copy() for parameter and return types
• Add parity test for function signature validation

The validation properly handles:

• Exact signature matches
• Untyped functions with typed annotations (dynamic compatibility)
• Numeric type compatibility (i32/i64/f32/f64)
• Async/non-async function distinction
• Optional parameters and defaults