Option to disable low-fidelity exception

This release adds the option to disable the limiter exception on low QUnit fidelity estimate, by setting environment variable QRACK_DISABLE_QUNIT_FIDELITY_GUARD to any truthy value.

When QUnit encounters a situation where acting a coupler gate would require exceeding user-specified maximum system memory resources, it will automatically resort to replacing this gate with a "classical shadow" that will not require additional allocation. In the process, QUnit will decrease its internal first-principles unitary fidelity estimation under assumption that the classical shadow replacement effect is worst-case. If the fidelity estimate drops below floating-point rounding epsilon, QUnit will automatically throw to exit simulation early. However, particularly in cases of "anti-concentration," such as with universal random circuit sampling, this first-principles worst-case fidelity estimate might be very overly severe. In these cases, the worst-case fidelity estimate cannot necessarily be trusted not to be (potentially very) overly pessimistic, but the throw condition on low fidelity estimate can be disabled by assigning any value to environment variable QRACK_DISABLE_QUNIT_FIDELITY_GUARD.

Full Changelog: vm6502qv9.12.23...vm6502q.v9.12.23

sha1sum results:
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