Polyfill: Replace unreachable retry with assertion in NudgeToCalendarUnit

[jessealama]

The retry path for negative durations appears unreachable: end-of-month constraining reduces the day, which for negative durations pushes the endpoint further from the origin, so the nudge window always contains the destination.

This isn't a rigorous proof of unreachability, but we haven't been able to construct a test case that exercises this path. Replace the retry block with an assertion guarding the invariant, so that if we're wrong, we'll get a clear error rather than silent misbehavior.

Closes #3235

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.57%. Comparing base (be4e0ef) to head (ff81f17).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3291      +/-   ##
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+ Coverage   97.40%   97.57%   +0.16%     
==========================================
  Files          22       22              
  Lines       10771    10757      -14     
  Branches     1866     1865       -1     
==========================================
+ Hits        10492    10496       +4     
+ Misses        258      240      -18     
  Partials       21       21              

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[jessealama]

This change was originally in #3289 (where it was simply deleted, without even an assertion in place).

[ptomato]

I think if we assume this code can't be reached by any JS code, we should change the spec text accordingly to have an assertion there as well, and submit it as an editorial change. (Could possibly also be done after stage 4.)

I have a hunch that the reason the nudge window only needs to be retried when moving forwards, is because the reason for the retry is when the edge of the window lands in a DST gap. The disambiguation: "compatible" behaviour always picks the later time for DST gaps, so that might be why. It's just a hunch, but maybe that helps ruling out why this doesn't happen?