Fix weekday modifiers after global calendar layout changes

[FanWu-ai]

Summary

Date and DateTime weekday modifiers use Monday-based weekday constants to index calendar.monthcalendar(). That function follows the standard library's process-wide first-weekday setting, so changing the calendar display layout also changes which dates Pendulum selects:

import calendar
import pendulum

calendar.setfirstweekday(calendar.SUNDAY)
pendulum.date(2026, 10, 15).first_of('month', pendulum.MONDAY)
# Before: Date(2026, 10, 4), a Sunday
# After:  Date(2026, 10, 5), a Monday

Use a separate Monday-based Calendar instance for the four month-calendar lookups. This fixes first_of, last_of, and the first occurrence in nth_of for month, quarter, and year units. The caller's global calendar setting is never changed. No API or dependency changes.

Tests

• Fourteen parametrized regression groups cover both classes and all seven global first-weekday settings. They check all requested weekdays, leap/non-leap February, month/quarter/year units, first/last/first-and-second occurrence, omitted weekday arguments, preserved timezones and midnight normalization, and state preservation after invalid arguments.
• Original source: 12 groups fail and the two Monday-layout controls pass. Fixed source: all 14 pass.
• Independent calendar oracle over every month in 2024 and 2025: 28,224 cases pass. The original source gives 18,144 incorrect dates for the same cases.
• An additional 1,000 concurrent queries return the correct dates without changing the caller's calendar setting.

Validation

Linux, CPython 3.12.14:

• Original full pure-Python and native suites: 1,854 passed, 3 existing skips each.
• Final full pure-Python and rebuilt native suites: 1,868 passed, 3 existing skips each.
• Repeated both complete suites against the built baseline/final release wheels in a clean runtime environment without typing_extensions: same passing results.
• Mypy: all 228 source files pass.
• Every configured prek hook passes, including Ruff, Rust formatting and Clippy.
• Strict MkDocs, release wheel/sdist builds, strict Twine checks, and git diff --check pass. Built Python source files were verified against the checkout.

The existing skips cover Windows-specific local-timezone behavior and two upstream timezone-rule tests. The documentation build retains six pre-existing info-level link diagnostics; source-distribution generation retains upstream Cargo metadata diagnostics. Other interpreters, operating systems, and hosted CI are not claimed as tested locally.

• Added tests for changed code
• Updated documentation

AI assistance was used to prepare the implementation and regression tests.