Fix mixed calendar and fixed-unit arithmetic across DST

[pranavpk404]

Fixes #838.

Summary

For aware datetimes, a mixed calendar/fixed-duration call currently applies all
units in naive local time. Reconstructing a time in a DST gap then normalizes
it forward, even when the requested fixed-duration shift was backwards.

For example, from 2024-04-01T04:00:00+03:00 in Europe/Sofia:

• subtract(days=1) reaches 2024-03-31T04:00:00+03:00.
• subtract(days=1, seconds=1) currently moves 3,599 seconds later.
• With this fix it reaches 2024-03-31T02:59:59+02:00, one second earlier.

Apply the existing calendar-only operation first, then the existing fixed-only
operation on the UTC timeline. This retains pure-calendar/fixed behavior and
the existing combined fractional rounding for naive datetimes. Document the
ordering for aware mixed-unit add and subtract calls.

Regression coverage

33 parametrized cases cover both methods and directions; days/weeks/months/years;
fractional seconds/calendar days; microseconds/minutes/hours; gaps and folds;
half-hour DST; multiple calendar units; UTC, naive and fixed-offset controls;
and subclasses. Elapsed-time expectations use independent stdlib arithmetic.

Validation

• Before the fix: 25 new regression cases failed, 8 controls passed.
• Updated upstream base 6394a10: full pure-Python suite 1882 passed, 3 skipped.
• Full newly built native suite: 1882 passed, 3 skipped.
• Native backend freshly rebuilt with locked PyO3 0.29.3 and Rust 1.83.0; activation verified.
• Full mypy: pass (229 files).
• prek run --all-files, including Ruff, Rust formatting and clippy: pass.
• git diff --check: pass.

Validated on Linux / CPython 3.14.7 with current locked dependencies.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

AI assistance was used for investigation, implementation and tests. The source
changes, independent reproduction, and regression expectations were reviewed
before submission.