ENH: Support array-like dataset backends

[oesteban]

Summary

• allow BaseDataset to accept array-like data (HDF5 datasets, memmaps) with tolerant repr/eq helpers
• preserve file-backed data when loading from HDF5 or NIfTI and optionally keep HDF5 handles open
• add backing-resource tracking/close helper for datasets

Testing

• python -m compileall -q src/nifreeze/data

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[codecov]

Codecov Report

❌ Patch coverage is 73.83178% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (62e5e43) to head (ec72e01).
⚠️ Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nifreeze/data/base.py	72.83%	17 Missing and 5 partials ⚠️
src/nifreeze/data/pet.py	77.77%	3 Missing and 1 partial ⚠️
src/nifreeze/data/dmri/base.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   79.62%   80.00%   +0.37%     
==========================================
  Files          30       30              
  Lines        1919     2005      +86     
  Branches      202      216      +14     
==========================================
+ Hits         1528     1604      +76     
- Misses        335      342       +7     
- Partials       56       59       +3     

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