Narrow zeroth-moment guard to the pow(0,0) corner case in parcel debug diagnostics

[Copilot]

parcel_test (Debug, none) was failing in unit_test_debug/test_spectrum.py::test_spectrum_diff[dradii_dr_dry] after a recent change in SD moment counting. The regression came from broad xp == 0 handling in the moment counter, which altered behavior beyond the intended pow(0,0) NaN-avoidance fix.

• Root-cause correction

  • In src/impl/diagnose_SD_attributes/particles_impl_moms.ipp, the fast path is now limited to the exact ambiguous case (xp == 0 && x == 0), instead of all xp == 0.

• Behavioral impact

  • Preserves the intended protection against pow(0,0) instability.
  • Restores previous numerical path for all nonzero x, avoiding unintended drift in debug spectrum outputs.

• Code change (minimal and localized)

const real_t n = thrust::get<0>(tpl);
const real_t x = thrust::get<1>(tpl);

if (xp == 0 && x == 0)
  return n;