Crumble: fix PropagatedPauliFrameLayer.toString when crossings contain objects [one liner]

[Stasiu51]

• What: Iterate crossings via {q1, q2} instead of [q1, q2] in PropagatedPauliFrameLayer.toString.
• Why: crossings entries are objects {q1, q2, color}. Array destructuring throws TypeError: .for is not
  iterable when calling String(layer) or String(pf).
• Reproducing from dev console in browser (which requires an un-minified version of crumble such as the one you get just by cloning stim and hosting locally):

const {Circuit} = await import('./circuit/circuit.js');
const {PropagatedPauliFrames} = await import('./circuit/propagated_pauli_frames.js');
const c = Circuit.fromStimCircuit(`
QUBIT_COORDS(0,0) 0
QUBIT_COORDS(1,0) 1
MARKX(0) 0
TICK
CX 0 1
`);
const pf = PropagatedPauliFrames.fromCircuit(c, 0);
String(pf); // throws in toString() today

[This bug was found with the help of AI tools]

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