Fix bad xgrid along zero longitude when LND grid not same as ATM

[nikizadehgfdl]

• Address issue Potential make_coupler_mosaic bug when using --land_mosaic #366
• When LND grid is not the same as ATM the xgrid is wrong along the Prime Meridian, E.g., if ATM is c96 and LND is c48, the xgrid cell areas are small by almost a factor of 2 along longitude=0 and the land mask has a spurious line along longitude=0
• The root cause is the Ocean grid is shifted right by 360 degree assuming that the land and atmos grids are the same which cause the atmXlnd exchange grid cells that are just to the left of the longitude=0 to be ignored.
• The fix is to (re)apply the fixlon to shift Ocean grid based on the atmxlnd_x[l][0] rather than the atm_avg.

Description
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Fixes # (issue)

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[nikizadehgfdl]

@ceblanton could you assign this to be reviewed (I don't seem to have permissions to do that). I suggest @mlee03 and @bensonr as reviewers. Thanks.

[ceblanton]

@nikizadehgfdl I added you to the github team that can write to the FRE repositories. Can you check?

[bensonr]

lgtm

[mlee03]

@nikizadehgfdl, could I request for the input and output files used to test this case? I want to make sure FMSgridtools/make_coupler_mosaic produces the correct xgrid

[nikizadehgfdl]

@mlee03, the input files are in

/work/Niki.Zadeh/exchange_grid_toolset/workdir/mosaic_ac96lc48oSPEAR.20260107.fixlon.an105/

so, the fix can be tested with the following command line on PAN (an105):

mpirun -np 6 /work/Niki.Zadeh/exchange_grid_toolset/tools/FRE-NCtools_main/builds/gfdlpan/bin/make_coupler_mosaic_parallel --atmos_mosaic /work/Niki.Zadeh/exchange_grid_toolset/workdir/mosaic_ac96lc48oSPEAR.20260107.fixlon.an105/C96_mosaic.nc --land_mosaic /work/Niki.Zadeh/exchange_grid_toolset/workdir/mosaic_ac96lc48oSPEAR.20260107.fixlon.an105/C48_mosaic.nc --ocean_mosaic /work/Niki.Zadeh/exchange_grid_toolset/workdir/mosaic_ac96lc48oSPEAR.20260107.fixlon.an105/ocean_mosaic.nc --mosaic_name mosaic_atmc96_lndc48_om4_SPEAR --ocean_topog /work/Niki.Zadeh/exchange_grid_toolset/workdir/mosaic_ac96lc48oSPEAR.20260107.fixlon.an105/ocean_topog.nc --verbose 2 --check