Add the years and months of a Period at once in LocalDate::plusPeriod()

[gnutix]

Item 2 of #137.

Problem

LocalDate::plusPeriod() called plusYears(), then plusMonths(), so a Feb 29 was clamped by both steps:

$d = LocalDate::of(2020, 2, 29);
$d->plusPeriod(Period::of(1, 1, 0));                   // 2021-03-28, but P13M gives 2021-03-29
$d->plusPeriod($d->until(LocalDate::of(2021, 3, 29))); // 2021-03-28: until() and plusPeriod() don't round-trip
LocalDate::of(999995, 2, 26)->plusPeriod(Period::of(5, -29, 5)); // throws "Invalid year: 1000000", the result 999997-10-01 is valid

Change

The years and months are added as a single number of months, as java.time does (plusMonths(toTotalMonths())), then the days. LocalDateTime::plusPeriod() and ZonedDateTime::plusPeriod() delegate to it, and minusPeriod() goes through it.

New cases in the providerPeriod of LocalDateTest and LocalDateTimeTest, used by both plusPeriod and minusPeriod.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Fm9bvgKFasEGzpK48bQrxA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.24%. Comparing base (44f8c01) to head (846b73e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #141   +/-   ##
=========================================
  Coverage     99.24%   99.24%           
  Complexity     1141     1141           
=========================================
  Files            48       48           
  Lines          2501     2501           
=========================================
  Hits           2482     2482           
  Misses           19       19           

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[BenMorel]

Thank you, @gnutix!