Improve parser error reporting clarity

[Codex]

Parsing failures previously only reported “unexpected token” without context. We now emit precise expectations alongside the offending token to make debugging grammar issues faster.

• Error diagnostics: Track expected token set during syntax matching and surface it with the unexpected token and location for clearer parse errors.
• Matching helpers: Added small helpers to normalize expected tokens and handle list/variant matches consistently.

Example error message shape:

unexpected token IDENTIFIER at 12:5, expected: COLON, EQUAL, SEMICOLON