fix: raise ParseError instead of AssertionError on unmatched '}'

[patchwright]

Problem

parser.parse("}") — an unmatched } — raises AssertionError from parser internals instead of the documented ParseError:

CLexer tokenizes RBRACE
  -> on_rbrace_func()
  -> CParser._lex_on_rbrace_func()
  -> CParser._pop_scope()
  -> assert len(self._scope_stack) > 1   # AssertionError

Callers that catch ParseError to handle parse failures cannot catch the AssertionError.

Cause

_pop_scope (c_parser.py:127) uses a bare assert to guard against an empty scope stack. Assertions are for invariant checks, not for user-input validation — and they vanish under python -O.

Fix

Replace the bare assert with an explicit ParseError:

def _pop_scope(self) -> None:
    if len(self._scope_stack) <= 1:
        raise ParseError("Unmatched '}'")
    self._scope_stack.pop()

Test

Added TestUnmatchedRbrace::test_unmatched_rbrace_raises_parse_error — fails on unpatched (AssertionError), passes with the fix. Full suite: 135 passed, 0 regressions (the 1 pre-existing test_examples failure is unrelated — AttributeError: Coord on master HEAD).

Fixes #603.

Assisted-by: Claude (Anthropic).