FIX: Validate required PET timing metadata when combining runs

[mnoergaard]

This follow-up hardens PET run combination for dual-time-window and other multi-run acquisitions.

1. Require valid TimeZero and finite numeric ScanStart, InjectionStart, and ImageDecayCorrectionTime for every run.
2. Reject runs whose frame timing begins before ScanStart.
3. Keep ScanStart as an internal consistency check rather than a cross-run alignment anchor.
4. Preserve the existing TimeZero/InjectionStart alignment and decay-rescaling calculations.
5. Add coverage for scan-relative and injection-relative dual-window timing.
6. Update the run-combination documentation to describe the required metadata and validation behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.19%. Comparing base (1617443) to head (f2ec05a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #336      +/-   ##
==========================================
+ Coverage   87.14%   87.19%   +0.04%     
==========================================
  Files          89       89              
  Lines       10552    10603      +51     
  Branches     1129     1136       +7     
==========================================
+ Hits         9196     9245      +49     
- Misses       1124     1125       +1     
- Partials      232      233       +1     

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