Fix LocalDateTime::plusDuration() when the nano carry crosses midnight

[gnutix]

Item 3 of #137.

Problem

LocalDateTime::plusDuration() / minusDuration() landed on the wrong day when adding the nanos carried a second across midnight:

LocalDateTime::of(2000, 1, 1, 23, 59, 59, 500_000_000)->plusDuration(Duration::ofMillis(600));
// 2000-01-01T00:00:00.1, expected 2000-01-02T00:00:00.1

LocalDateTime::of(2000, 1, 1, 0, 0, 0, 500_000_000)->minusDuration(Duration::ofMillis(400));
// 1999-12-31T00:00:00.1, expected 2000-01-01T00:00:00.1

The day shift was computed from the seconds only, while LocalTime::plusDuration() applies the nano carry. The second case comes from how a negative fractional duration is stored: -0.4 s is (-1 s, +600,000,000 ns). This only happens when both the date-time and the duration have a fractional second.

Change

The day shift now includes the carry, intdiv(nano + duration nanos, NANOS_PER_SECOND). It is never negative, because both nano values are in [0, 1e9).

There are two new cases in providerDuration, which both testPlusDuration and testMinusDuration use: one crossing midnight forwards and one backwards.

🤖 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 (74e4c95).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #142   +/-   ##
=========================================
  Coverage     99.24%   99.24%           
  Complexity     1141     1141           
=========================================
  Files            48       48           
  Lines          2501     2503    +2     
=========================================
+ Hits           2482     2484    +2     
  Misses           19       19           

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[BenMorel]

Thank you, @gnutix!