Fix cached-quantity rollback after exceptions

[steven-murray]

Summary

• make first-time cached quantity evaluation rollback partial cache bookkeeping on exceptions
• factor the success-path bookkeeping into a helper to keep cache indexing explicit
• add regressions for recovery after update and full transactional cleanup

Fixes #92

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.86%. Comparing base (14cdf37) to head (d32f74e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
+ Coverage   97.83%   97.86%   +0.02%     
==========================================
  Files          26       26              
  Lines        2634     2671      +37     
==========================================
+ Hits         2577     2614      +37     
  Misses         57       57              

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