fix: remove_idle_qubits KeyError on shared operand nodes

[SAY-5]

Summary of changes

Closes #331.

unroll() can emit gate statements that share the same IndexedIdentifier operand node (the crz decomposition does this), and _remap_qubits rewrites bit.indices[0][0].value in place per statement, so a node shared by k statements gets remapped k times. With a lower-indexed idle qubit the repeated lookups walk off the index map and raise KeyError, e.g. crz(0.5) q[1], q[2]; on a 3-qubit register.

This tracks visited node ids so each operand node is remapped exactly once, and adds a regression test with the minimal repro from the issue. The remap now also produces correct indices for shared nodes rather than silently double-remapping them when the shift happens to stay inside the map.

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