ENH: X5 read/write support of TransformChain

[oesteban]

Summary

• enable writing individual transforms and paths for transform chains in X5 files
• implement loading of X5 chains
• verify deduplicated appending and round‑trip behaviour

Testing

• pytest nitransforms/tests/test_manip.py::test_transformchain_x5_roundtrip -q

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https://chatgpt.com/codex/tasks/task_e_687cc0babcd48330b596a86a467e9d82

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 9 lines in your changes missing coverage. Please review.

Project coverage is 94.79%. Comparing base (fc6fbbd) to head (8ced1ac).

Files with missing lines	Patch %	Lines
nitransforms/manip.py	90.00%	2 Missing and 3 partials ⚠️
nitransforms/linear.py	81.81%	1 Missing and 1 partial ⚠️
nitransforms/nonlinear.py	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   95.09%   94.79%   -0.31%     
==========================================
  Files          16       16              
  Lines        1918     1977      +59     
  Branches      251      267      +16     
==========================================
+ Hits         1824     1874      +50     
- Misses         75       79       +4     
- Partials       19       24       +5     

Flag	Coverage Δ
unittests	94.61% <91.66%> (-0.31%)	⬇️

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