Polyfill: Fix extreme PlainYearMonth crash in non-ISO calendars

[jessealama]

calendarToIsoDate()'s bisection loop can produce intermediate ISO dates beyond the legacy Date boundary, causing isoToCalendarDate() to fail via Intl.DateTimeFormat.formatToParts().

The fix is to override isoToCalendarDate in affected calendar helpers to (1) shift out-of-range ISO dates by a calendar-appropriate cycle, (2) convert, then (3) adjust the year back. Another wrinkle is that Coptic/Ethiopian and Islamic calendars use day-based shifting with exact cycle lengths to avoid Julian–Gregorian drift; Indian uses year-based shifting since its months start at fixed Gregorian dates; Hebrew doesn't need an override since its extreme dates stay within range during bisection, though I'm not fully sure this holds in all cases.

Fixes #3251

[codecov]

Codecov Report

❌ Patch coverage is 93.42105% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.93%. Comparing base (8827087) to head (9b223e5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
polyfill/lib/calendar.mjs	93.42%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3279      +/-   ##
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- Coverage   97.96%   97.93%   -0.04%     
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  Files          22       22              
  Lines       10519    10586      +67     
  Branches     1814     1827      +13     
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+ Hits        10305    10367      +62     
- Misses        197      201       +4     
- Partials       17       18       +1     

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[ptomato]

This seems reasonable. But the approximate nature of the shifts for some calendars makes me uneasy, and I find it hard to reason about.

One alternative idea that I had, would be to extend the range beyond the legacy Date range more conservatively, staying only within the same month. e.g., if new Temporal.PlainDate(-271821, 4, 19, 'islamic-civil') gives { year: -280804, month: 3, day: 21 }, then we can manually calculate the calendar date for ISO dates between -271821-03-30 and -271821-04-18.

No idea if this would be sufficient to not crash in the bisection loop; maybe we need to bisect with dates outside of that, in which case we should probably go with the solution currently in this PR.

[ptomato]

I'd still like to try the approach I suggested, but given that we don't have a lot of time, better to merge this since it works and iterate later. Thanks again!