Reject bare expression statements instead of raising AttributeError

[SAY-5]

Summary of changes

Closes #388.

A bare expression statement such as a * b; is valid OpenQASM 3 and appears throughout the spec's classical types examples. The visitor's dispatch table routed every ExpressionStatement straight to _visit_function_call, which dereferences .name on the expression. For a non-Identifier expression that raised AttributeError: 'BinaryExpression' object has no attribute 'name', which escaped the public API.

Expression statements have no effect on the quantum program, so they are now rejected with a ValidationError reading Expression statement has no effect, carrying the source span, rather than crashing. Function-call statements keep their existing behaviour.

Added a regression test in tests/qasm3/test_expressions.py and a CHANGELOG entry.

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