Fix valid-epoch RV results and cache ownership

[SuperdoerTrav]

Carry valid epochs through terminated numerical RV batches, preserve caller-owned cache inputs, and validate integration steps and tolerances.

Verified: 13 patch integration tests passed; Ruff E9/F63/F7/F82, compileall, and git diff --check passed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.75258% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.32%. Comparing base (37373cc) to head (18e7505).

Files with missing lines	Patch %	Lines
ssapy/propagator.py	90.24%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
- Coverage   97.37%   97.32%   -0.06%     
==========================================
  Files          17       17              
  Lines        5826     5885      +59     
==========================================
+ Hits         5673     5727      +54     
- Misses        153      158       +5     

Flag	Coverage Δ
unittests	97.32% <91.75%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
ssapy/compute.py	94.92% <100.00%> (+0.11%)	⬆️
ssapy/propagator.py	98.41% <90.24%> (-0.73%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.