Enhancement: Introduce Result type for parser error handling

[clank-hayen]

Problem

The parser currently uses exceptions for error handling:

throw runtime_error("expected 'or' handler (return, fail, match, block, continue, or break) at line " + to_string(or_line));

This approach has issues:

1. Control flow is hard to follow - exceptions can propagate unexpectedly
2. No recovery possible - the parser stops at first error
3. Inconsistent with codegen - which uses C++ exceptions but typechecker collects errors

Current State

• Lexer: Throws `runtime_error` on invalid input
• Parser: Throws `runtime_error` on syntax errors
• Typechecker: Collects errors in `TypeCheckerState.errors` vector (good pattern!)
• Codegen: Returns strings, no explicit error handling

Suggested Solution

Adopt a Result pattern similar to what typechecker does:

// Option 1: Use expected-like type
template<typename T>
using ParseResult = std::expected<T, ParseError>;

// Option 2: Follow typechecker pattern
struct ParserState {
    vector<ParseError> errors;
    // ...
};

// Continue parsing after errors
void report_error(ParserState& state, const string& msg, int line) {
    state.errors.push_back({msg, line, state.filename});
    synchronize(state);  // Skip to next statement
}

This would allow:

• Collecting multiple errors in a single parse
• Better error recovery
• Consistent error handling across the compiler

Implementation Notes

• Start with parser, extend to lexer later
• Add synchronization points to recover from errors
• Follow the existing typechecker pattern

Priority

Medium - Current exception-based approach works but limits error reporting quality.