Malformed cvar_export is misreported by native and silently accepted by the Python interpreter #1353

A malformed cvar_export that uses variable instead of the required variables array is handled differently by each path:

src = {"format": "PHIR/JSON", "version": "0.1.0", "ops": [
    {"data": "cvar_define", "data_type": "u32", "variable": "m", "size": 2},
    {"data": "cvar_export", "variable": "m"},
]}

• Native (run_phir_sim): Failed to parse PHIR JSON: missing data_type. The AST decodes it as a variable definition, so the message points at the wrong field.
• HybridEngine with the Rust interpreter: Failed to parse: missing data_type.
• HybridEngine with the Python interpreter (phir_validate=False): accepted silently, returns {'m': ['00']}.

Direction

Reject a data directive that is not a declaration and has no variables array on every path, with an error naming the directive and the missing variables field. On the #1314 branch the AST already does this when variable is also missing. Stop decoding unknown or non-declaration data ops as VariableDefinition.

Found during the review of the #1314 work.