Export parameterized circuits to OpenQASM 3

[alyabouzaid]

Closes #146.

to_qasm3() already prints parameter symbols inside gate arguments, but it never declared them, so the OpenQASM 3 for any parameterized circuit was invalid: it referenced symbols that don't exist. For example a circuit with rx(theta) and ry(phi) produced output that started straight at qubit[2] q; and then used theta/phi with no declaration.

This follows the approach laid out in the issue discussion: add a small QuantumCircuit::parameter_symbols() stab and feed its result to the exporter, so the C++ side does not keep a second copy of the parameter list that lives on the Rust side.

Changes

• QuantumCircuit::parameter_symbols() returns the free parameter symbol names used by the circuit, in first-encountered order. The C API currently exposes only the count of symbols (qk_circuit_num_param_symbols), not the symbols themselves, so the names are recovered by scanning the instruction parameters. This is the single place to swap in a C API call once per-symbol accessors exist.
• to_qasm3() now emits one input float[64] <name>; per symbol, after the gate definitions and before the register declarations, so every referenced symbol is declared.

OPENQASM 3.0;
include "stdgates.inc";
input float[64] theta;
input float[64] phi;
qubit[2] q;
rx(theta) q[0];
ry(phi) q[1];

Limitations (from the C API, not this change)

• Gate arguments must be bare symbols. A compound expression such as 2*theta can't be reduced to a single input name, so it raises std::runtime_error instead of emitting an invalid identifier.
• Two distinct Parameter objects that share a name are merged into one symbol, because the C API tracks symbols by name and gives no UUID to tell them apart. The recovered names are cross-checked against qk_circuit_num_param_symbols as a guard.

Both limitations are noted in the method docs and a release note.

Tests

Added test_parameter_symbols, test_to_qasm3_parameterized, and test_to_qasm3_shared_parameter to test_circuit.cpp. Full suite passes locally (Qiskit 2.4.1 C extension, macOS):

100% tests passed, 0 tests failed out of 4

🤖 Generated with Claude Code

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[1ucian0]

Closing in favor of #159

Thanks @alyabouzaid for the contribution anyway! If you would like to work on another issue, take a look at https://qisk.it/good-first-issues for a list of newcomer-friendly issues.