WIP: Add configurable longitudinal anatomical reference processing

[mnoergaard]

This PR addresses issue #300, updating the longitudinal processing by replacing the old boolean --longitudinal behavior with a new --subject-anatomical-reference option supporting:

1. first-lex: default, aligns anatomical references to the first lexicographic T1w image

2. unbiased: builds an unbiased longitudinal template, equivalent to the deprecated --longitudinal

3. sessionwise: builds independent anatomical workflows per session

The workflow now creates explicit subject/session processing groups, applies session-specific BIDS filters when running sessionwise, names session workflows distinctly, and threads session-aware FreeSurfer subject IDs into the anatomical workflow. The deprecated --longitudinal flag remains available as an alias for --subject-anatomical-reference unbiased.

This also adds config serialization support for processing groups, updates documentation for longitudinal/sessionwise behavior, and expands tests around CLI parsing, config round-tripping, sessionwise workflow construction, and Nipype-safe subject/session ID generation.

[codecov]

Codecov Report

❌ Patch coverage is 98.42105% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.30%. Comparing base (d4193e2) to head (e366441).

Files with missing lines	Patch %	Lines
petprep/cli/parser.py	95.45%	0 Missing and 1 partial ⚠️
petprep/config.py	97.14%	0 Missing and 1 partial ⚠️
petprep/workflows/base.py	97.36%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
+ Coverage   83.69%   84.30%   +0.61%     
==========================================
  Files          87       87              
  Lines        8600     8776     +176     
  Branches      916      946      +30     
==========================================
+ Hits         7198     7399     +201     
+ Misses       1174     1147      -27     
- Partials      228      230       +2     

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