Support parameterized circuits in QASM3 export

[spital]

Summary

Closes #146.

This moves the OpenQASM 3 export implementation out of QuantumCircuit into a dedicated exporter while keeping the public QuantumCircuit::to_qasm3() API. It also adds support for exporting parameterized circuits by emitting symbolic parameters as OpenQASM 3 input float[64] declarations.

What changed

• Add a dedicated OpenQASM 3 exporter in src/circuit/qasm3_exporter.hpp and delegate QuantumCircuit::to_qasm3() to it.
• Harvest symbolic identifiers from circuit parameter expressions and emit them as input float[64] declarations before registers and operations.
• Allocate emitted input/register names so they avoid OpenQASM keywords, stdgates.inc globals, and custom gate definitions emitted by the exporter.
• Preserve the existing flattened q register export behavior.
• Emit valid OpenQASM 3 for controlled-U1, controlled-U3, and non-zero global phase (gphase).
• Add regression tests for parameterized export, identifier collisions, custom-gate collisions, controlled-U definitions, and global phase.

Known limitations

The current Qiskit C API exposes the number of circuit parameter symbols but not an iterator over their names, so the exporter derives parameter identifiers from the string form returned by qk_param_str(). This supports normal symbolic parameters and expressions, but parameter names should be OpenQASM-identifier-like.

Some expression forms emitted by Qiskit's parameter string representation, such as power expressions (theta**2) or function calls (sin(theta)), are not currently round-trippable through qiskit_qasm3_import. The exporter preserves the expression spelling instead of rewriting it to a form that may change semantics.

Testing

• make -C test/deps/qiskit c
• cmake -S test -B /tmp/qisk146-hermes-review60-build
• cmake --build /tmp/qisk146-hermes-review60-build -j20
• ctest --test-dir /tmp/qisk146-hermes-review60-build --output-on-failure
• /tmp/qisk146-hermes-review60-build/test_driver test_circuit
• git diff --check origin/main...HEAD
• Parser smoke tests with qiskit 2.4.1 / qiskit-qasm3-import 0.6.0 using qiskit.qasm3.loads() for reserved-name conflicts, keyword/constant-name conflicts, classical-register conflicts, controlled-U definitions, and global phase output.

AI assistance disclosure

I used Codex, Claude, and Hermes as AI assistants to help prototype, review, compare related approaches, and draft PR text. I manually reviewed the resulting diff, checked the live GitHub issue/PR state and unitaryHACK policy, refined the implementation, and ran the tests listed above locally. The PR is submitted with human oversight; any remaining mistakes are mine.

• I have added the tests to cover my changes.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.