"Near-Clifford rounding" fidelity calculation

QUnit already offered GetUnitaryFidelity() (or, in PyQrack, get_unitary_fidelity()) to automatically estimate overall fidelity reduction during SDRP algorithm application. Similarly, when using using Clifford+RZ gate set, QStabilizerHybrid can be instructed via QRACK_NONCLIFFORD_ROUNDING_THRESHOLD environment variable to round non-Clifford phase gates with smaller angles to "no-operation," but it wasn't immediately obvious how this should quantitatively affect overall fidelity. Now, under the assumption that non-Clifford phase gates will only be buffered at all when applied to states that are originally in maximal superposition, Qrack automatically calculates the effect on the single-qubit reduced density matrix:

fidelity *= 0.25 * norm(1.0 + phaseFac)

where phaseFac is the unit-norm phase factor of the gate "rounded away." This produces a maximum infidelity for a T gate, of 3/8 compounded infidelity. (Note that, if you're using variational RZ gates in an RCS benchmark, compounded infidelity per rounding event might typically be much less than 3/8, and it's quite possible to set the rounding parameter to maximum yet still see approximately 1.0 fidelity at the end of the circuit.)

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