feat(mfusg): add Transient IBOUND (TIB) package

[reneangermeyer]

The TIB package allows changing IBOUND and ICBUND values at any stress period in MODFLOW-USG. Use cases include excavation/reclamation, well drilling/plugging, and transient prescribed head/concentration cells. FloPy currently has no TIB support.

Supports six data categories per stress period: ib0, ib1, ibm1 (IBOUND) and icb0, icb1, icbm1 (ICBUND). Node arrays (ib0/icb0) use U2DINT format via Util2d, while list records (ib1/ibm1/icb1/icbm1) support HEAD/AVHEAD and CONC/AVCONC keywords. As required by the TIB specification, all node numbers are global — for structured grids, model.dis.get_node() can be used to convert (layer, row, col) tuples.

Changes:

• flopy/mfusg/mfusgtib.py: new MfUsgTib(Package) class with write_file, load, and get_empty
• flopy/mfusg/init.py: add import and all entry, remove pre-existing duplicate MfUsgGnc entry
• flopy/mfusg/mfusg.py: register TIB in mfnam_packages
• autotest/test_mfusg_tib.py: 10 tests covering round-trip, transport data, empty stress periods, input validation, and both DIS and DISU grids

[codecov]

Codecov Report

❌ Patch coverage is 90.55556% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (1c57750).
⚠️ Report is 136 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mfusg/mfusgtib.py	90.5%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2724      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      676      +32     
  Lines       124135   130920    +6785     
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+ Hits         68947    94915   +25968     
+ Misses       55188    36005   -19183     

Files with missing lines	Coverage Δ
flopy/mfusg/__init__.py	100.0% <100.0%> (ø)
flopy/mfusg/mfusg.py	78.3% <ø> (+0.8%)	⬆️
flopy/mfusg/mfusgtib.py	90.5% <90.5%> (ø)

... and 569 files with indirect coverage changes

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