`Circuit.from_graph` changes the measurement basis after ZX colour conversion

[WindOctober]

Describe the issue

Converting a measurement circuit to a graph, applying simplify.to_gh, and converting it back adds an unwanted Hadamard before the measurement. This changes the measurement basis: on input |0⟩, the original circuit always measures 0, while the extracted circuit measures 0 or 1 with equal probability.

The issue requires only ZX colour conversion, not a simplification pass.

Explain how to reproduce the bug or problem

import pyzx as zx
from pyzx import simplify

source = """OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
bit[1] c;
c[0] = measure q[0];
"""
circuit = zx.Circuit.from_qasm(source)
graph = circuit.to_graph()

print("original:", [str(g) for g in circuit.gates])
print("roundtrip:", [str(g) for g in zx.Circuit.from_graph(graph).gates])
simplify.to_gh(graph)
print("after to_gh:", [str(g) for g in zx.Circuit.from_graph(graph).gates])

Observed output:

original: ['Measurement(0)']
roundtrip: ['Measurement(0)']
after to_gh: ['HAD(0)', 'Measurement(0)']

All three should represent the same measurement, without an added Hadamard. A related manifestation is that H; measure becomes measure after clifford_simp and extraction.

Version

PyZX 0.10.6, Python 3.10.14, Linux. Also reproduced on master at ad022cf1d9 (2026-09-17).

Root cause

to_gh converts the X-type measurement leaf to Z-type and toggles its incident edge. This preserves the graph semantics, but graph_to_circuit emits a plain Measurement without accounting for the leaf's colour, leaving an extra edge-induced Hadamard. Extraction should account for both the leaf colour and the edge type.

[a6hinandh]

Hi! I'd like to take this one. I can reproduce it on master.

The measurement leaf's edge isn't part of the qubit wire, but graph_to_circuit emits a HAD for it whenever it's a Hadamard edge, which is what happens after to_gh recolours the X leaf. My plan is to skip that edge and read the basis from the colours instead: seen from the on-wire spider, a Hadamard edge flips the leaf's colour, so a Z on-wire spider gives a Z-basis measurement and an X on-wire spider gives an X-basis one (H; measure; H). I have a fix with a regression test locally and will open a PR shortly.