FIX: solve HMC-off identity transform collisions across PET series

[mnoergaard]

This fixes a --hmc-off failure mode where identity HMC transforms were written to a shared idmat.tfm path in the work directory. For multi-session or multi-run datasets, that file could be overwritten by another PET series with a different frame count, causing downstream resampling to crash when the transform count no longer matched the PET volume count.

Changes:

1. Write HMC-off identity transforms to PET-series-specific filenames.
2. Use the PET image frame count when generating identity transforms.
3. Add an explicit transform-count validation error in resampling.
4. Add regression tests for multi-series identity transform isolation and transform-count mismatch handling.

[codecov]

Codecov Report

❌ Patch coverage is 98.57143% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.40%. Comparing base (8a48d90) to head (27ac758).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
petprep/interfaces/resampling.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
+ Coverage   86.24%   86.40%   +0.15%     
==========================================
  Files          88       89       +1     
  Lines        9988    10052      +64     
  Branches     1056     1060       +4     
==========================================
+ Hits         8614     8685      +71     
+ Misses       1143     1134       -9     
- Partials      231      233       +2     

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