feat: support mf6 binary output file conversion

[wpbonelli]

Classic MODFLOW programs (NWT, MF2005, MF2000) produce binary output files that MF6 post-processors (e.g. PRT) cannot directly consume, because the budget format is fundamentally different and MF6 requires a binary grid file that classic programs do not write. Add conversion capability so classic program output can be converted to MF6 format and used with MF6 transport or particle-tracking models.

Introduce ClassicMfToMf6Converter that reads a classic compact head/budget file pair and writes three files

• .hds — copies verbatim
• .bud — converts FLOW RIGHT/FRONT/LOWER FACE → FLOW-JA-FACE + DATA-SAT
• .grb — creates binary grid file from DIS geometry

This builds on #2722 and required a few more utilities

• get_icelltype_from_laytyp(): convert NWT LAYTYP to MF6 ICELLTYPE
• get_icelltype_from_laycon(): convert BCF LAYCON to MF6 ICELLTYPE
• get_saturation(): calculate saturation from head, cell geometry, cell type
• get_structured_flowja(): inverse of get_structured_faceflows()
• MfGrdFile.write_dis(): write grb file for structured grid

Usage

converter = ClassicMfToMf6Converter.from_model(mf, "model.hds", "model.cbc")  # from model
converter = ClassicMfToMf6Converter("model.hds", "model.cbc", nlay, nrow, ncol, ...)  # from files
converter.convert("mf6_output/")

Limitations

• Requires compact budget output (COMPACT BUDGET in OC)
• Structured grids only (DIS)
• DATA-SPDIS not included yet

[codecov]

Codecov Report

❌ Patch coverage is 78.71287% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (19cd49c).
⚠️ Report is 152 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/classic_to_mf6.py	73.5%	37 Missing ⚠️
flopy/mf6/utils/binarygrid_util.py	74.7%	27 Missing ⚠️
flopy/utils/postprocessing.py	80.9%	12 Missing ⚠️
flopy/mf6/utils/postprocessing.py	89.1%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2746      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +16.9%     
===========================================
  Files          644      676      +32     
  Lines       124135   132800    +8665     
===========================================
+ Hits         68947    96312   +27365     
+ Misses       55188    36488   -18700     

Files with missing lines	Coverage Δ
flopy/mf6/utils/__init__.py	100.0% <100.0%> (ø)
flopy/utils/__init__.py	100.0% <100.0%> (ø)
flopy/mf6/utils/postprocessing.py	92.5% <89.1%> (-4.3%)	⬇️
flopy/utils/postprocessing.py	80.2% <80.9%> (+2.6%)	⬆️
flopy/mf6/utils/binarygrid_util.py	86.5% <74.7%> (-4.3%)	⬇️
flopy/utils/classic_to_mf6.py	73.5% <73.5%> (ø)

... and 571 files with indirect coverage changes

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