[Zarr-Support] Introduce LazyArray abstraction for HDF5-backed DataStub

[ehennestad]

Summary

Create a LazyArray base type and a HDF5LazyArray backend for the DataStub class. This moves all h5 specific code out of the DataStub and prepares for adding support to read Zarr files .

What Changed

• add io.backend.base.LazyArray
• add io.backend.hdf5.HDF5LazyArray
• refactor types.untyped.DataStub to delegate metadata lookup and lazy loads through a backend lazy array
• update io.backend.hdf5.HDF5Reader to wrap non-scalar non-empty datasets in DataStub backed by HDF5LazyArray
• extend backend tests and add focused HDF5LazyArray coverage

[codecov]

Codecov Report

❌ Patch coverage is 92.46862% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.42%. Comparing base (4387700) to head (26cfedf).

Files with missing lines	Patch %	Lines
+io/+backend/BackendFactory.m	47.05%	9 Missing ⚠️
+io/+backend/+hdf5/@HDF5LazyArray/load_mat_style.m	94.53%	7 Missing ⚠️
+io/+backend/+base/LazyArray.m	96.96%	1 Missing ⚠️
+io/+backend/+hdf5/@HDF5LazyArray/load_h5_style.m	96.15%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           refactor-hdf5-backend-reader     #800      +/-   ##
================================================================
- Coverage                         95.51%   95.42%   -0.09%     
================================================================
  Files                               195      199       +4     
  Lines                              7151     7208      +57     
================================================================
+ Hits                               6830     6878      +48     
- Misses                              321      330       +9     

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