src_compressible_stag: reduce data_xcross across ranks in the all_correl branch

[isriva]

evaluateStatsStag1D has two paths for gathering the cross-cell pencil data. The
all_correl == 0 path reduces data_xcross across ranks before using it; the
all_correl == 1 path did not.

GetPencilCross assigns into the flat buffer only for cells where i == x_star, so
each rank touches only the (j,k) slots of boxes it owns that contain that plane.
Without the reduction every other rank saw zeros, and
EvaluateSpatialCorrelations1D divides by those entries (1.0/cvcross,
1.0/(meanrhocross*meanrho)), producing inf/NaN rather than merely wrong numbers.

The buffer was also constructed once outside the loop over the five x* values
and never re-zeroed, so iterations 1-4 additionally inherited stale entries from
the previous x*. Moving the declaration inside the loop fixes both.