[MORTEM3845]

Adds a single optional IsTimeAllowed property (Predicate<DateTime>?) to TimePicker and Clock, allowing applications to decide which minute slots can be selected. A null rule preserves unrestricted selection.

For example, appointments can be available from 06:00 through 19:00, every 10 minutes, with a 13:00-14:00 lunch break, using only this property:

picker.IsTimeAllowed = time =>
    time.TimeOfDay >= TimeSpan.FromHours(6)
    && time.TimeOfDay <= TimeSpan.FromHours(19)
    && time.Minute % 10 == 0
    && time.Hour != 13;

The native clock keeps its existing layout and disables unavailable values. The same rule applies to dragging, typed input, and programmatic selection. Programmatic values are coerced to the nearest allowed minute on the same day; invalid typed input follows the existing invalid-text behavior.

Rules receive the selected date and time with seconds cleared: availability is defined per minute, while WithSeconds continues to control seconds selection. If captured application data changes, call RefreshTimeAvailability() to reevaluate the current selection and clock buttons. If no minute is allowed, TimePicker.SelectedTime becomes null and clock choices are disabled.

The existing MinuteSelectionStep behavior is preserved. No additional range or increment properties are introduced.

Addresses #3733.

Validation:

• Release build for net462, net8.0-windows, and net10.0-windows.
• Full MaterialDesignThemes.Wpf.Tests suite: 402 passed, 1 pre-existing skipped test, 0 failures.
• New coverage for range endpoints, lunch breaks, minute steps, manual input and its selected date, dragging, AM/PM consistency, dynamic availability, and empty availability.