Duplicate R3bar accumulation line doubles bubble perturbation

[sbryngelson]

Bug

In `src/pre_process/m_assign_variables.fpp`, the R3bar moment accumulation line is duplicated, causing the bubble radius cubed moment to be accumulated twice per iteration.

File: `src/pre_process/m_assign_variables.fpp`, lines 235-236

Impact: R3bar is double-counted, producing incorrect bubble volume fraction perturbations during pre-processing initialization with polydisperse bubbles.

Severity: High — bubble perturbation initialization computes wrong moment values.

[sbryngelson]

Fix: #1186