Preserve endpoint folds when calculating interval components

[FanWu-ai]

Interval.__init__ reconstructs the endpoints for precise_diff without their fold values. This changes the UTC offset during a repeated DST hour, even though Interval.__new__ preserves the fold for the total duration.

For example, New York on 2024-11-03, 01:15 with fold=0 to 01:45 with fold=1, currently has total_seconds() == 5400 but reports (hours, minutes) == (0, 30) and in_words() == "30 minutes".

Pass each original fold to the corresponding reconstruction, matching __new__. The interval then correctly reports one hour and thirty minutes. Tests cover both endpoints, signed/reversed/absolute intervals, and same-fold controls.

Pull Request Check List

• Added tests for changed code: 12 failures on the old implementation in each backend; 20 new cases including controls
• Full native-extension suite: 1870 passed, 3 upstream skips
• Full pure-Python suite: 1870 passed, 3 upstream skips
• Mypy: 228 source files, no issues
• All prek hooks, including Rust formatting/clippy, passed
• Documentation: no API change; preserving existing endpoint metadata

The untouched baseline passed the same checks before editing. Validation used Python 3.12.14, the real Rust extension, and locked dependencies. This is a fold-preservation fix; the separate same-wall-time datetime comparison issue discussed in #351 is outside its scope.

Independently reviewed and rechecked with both full backends, mypy, and every prek hook.

Prepared with OpenAI Codex assistance.