Native PHIR-JSON engine ignores cvar_export and returns every register #1323

The native PHIR-JSON engine ignores cvar_export, so sim(PhirJson(...)) returns every classical register instead of only the exported ones.

Cause (from source on the #1314 branch; same on dev)

The PHIR v0.1 spec (crates/pecos-phir-json/specification/v0.1/spec.md) defines {"cop": "cvar_export", "args": [...], "to": [...]} as selecting which classical variables are returned, optionally renaming them with to. In PECOS:

• the native engine's DataExport handling skips it (crates/pecos-phir-json/src/v0_1/engine.rs, DataExport arm, and the block executor), and get_results returns all classical registers when no Result mapping exists;
• the AST does not keep the to field.

A regression added in the #1314 work shows it: with registers m and scratch, X then Measure into m, and cvar_export ["m"], the native engine returns both m and scratch.

This also blocks the PHIR JSON -> pecos-phir module path: lowering cvar_export to the module's Result instructions would follow the spec (return only m) and disagree with the native engine, so after #1314 the converter rejects cvar_export with a named error. QASM -> PHIR and QuantumCircuit.to_phir_json() both emit cvar_export, so most real programs cannot use the module path until this is fixed.

Direction

1. Make the native engine honour cvar_export per the spec: return only exported variables, renamed by to when given (keep the AST field). Programs without any cvar_export keep returning all registers.
2. Lower cvar_export to module Result instructions in phir_json_to_module, and test that QASM-with-creg and qc2phir programs give identical results through the native engine and JSON -> module -> PhirEngine.
3. This changes what sim(PhirJson(...)) returns for programs with exports (unexported registers disappear): list it as a breaking change, and check PECOS's own producers and tests that rely on unexported registers.