Polyfill: Replace check with assertion in calendarDateWeekOfYear

[catamorphism]

(lastRelDow < 0) can never be true, so replace the check with an assertion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.78%. Comparing base (03f9057) to head (a8fbdb0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3213      +/-   ##
==========================================
+ Coverage   96.75%   96.78%   +0.02%     
==========================================
  Files          22       22              
  Lines       10398    10398              
  Branches     1859     1858       -1     
==========================================
+ Hits        10061    10064       +3     
+ Misses        289      286       -3     
  Partials       48       48              

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