Polyfill: Change some checks to assertions

[catamorphism]

Some checks in eraFromYear must always be true; change exception-throwing code to assertions and document why the assertions are always true.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.77%. Comparing base (03f9057) to head (e725081).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3215      +/-   ##
==========================================
+ Coverage   96.75%   96.77%   +0.02%     
==========================================
  Files          22       22              
  Lines       10398    10402       +4     
  Branches     1859     1857       -2     
==========================================
+ Hits        10061    10067       +6     
+ Misses        289      287       -2     
  Partials       48       48              

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