Implement delivery appointment checks

Create DeliveryAppointments.java. Assume Java 24.

Implement:

List<Appointment> schedulableAppointments(Planner planner)

Rules:

• planner.appointments() returns appointments in the planner's proposed order.
• CalendarService.canSchedule(appointment.token()) is a blocking remote call.
• A planner can contain hundreds of appointments, and checking one appointment at a time is too slow in production.
• The concurrency limit is per schedulableAppointments(planner) call: during one call, run at most 8 in-flight CalendarService.canSchedule(...) calls at the same time.
• Return only appointments that can be scheduled.
• Sort the returned appointments by Appointment::startsAt, then Appointment::token.
• Use Java stream APIs for the operation.

Use these nested types:

record Planner(List<Appointment> appointments) {}
record Appointment(String token, long startsAt) {}
static final class CalendarService {
    static boolean canSchedule(String token) {
        throw new UnsupportedOperationException("provided by production");
    }
}