cs and csdg from stdgates.inc are rejected as unsupported operations

[ryanhill1]

Summary

cs and csdg are in OpenQASM 3's stdgates.inc, but pyqasm rejects both at unroll with ValidationError: Unsupported / undeclared QASM operation: cs. The other controlled standard gates (ch, cy, cz, crx, …) unroll fine.

Repro (pyqasm 1.2.1)

import pyqasm
for g in ("cs", "csdg", "ch"):
    m = pyqasm.loads(f'OPENQASM 3.0;\ninclude "stdgates.inc";\nqubit[2] q;\nbit[2] c;\n{g} q[0], q[1];\nc = measure q;\n')
    m.unroll()
# cs   -> ValidationError: Unsupported / undeclared QASM operation: cs
# csdg -> ValidationError: Unsupported / undeclared QASM operation: csdg
# ch   -> OK

Impact

Seen in production through qbraid-qir (QASM->QIR conversion failed: Unsupported / undeclared QASM operation: csdg) on the qBraid simulators, 6 jobs from one user between 2026-09-23. Any Qiskit circuit containing CSGate/CSdgGate exported to OpenQASM 3 hits it.

Expected

Both decompose trivially: cs a, b = cp(pi/2) a, b; csdg a, b = cp(-pi/2) a, b.