Implement offer availability filtering

Use $java-streams to create OfferAvailability.java. Assume Java 24.

Implement:

List<Offer> availableOffers(List<Offer> offers)

Rules:

• AvailabilityApi.lookup(offer.id()) is a blocking remote call.
• During one availableOffers(offers) call, run at most 8 in-flight AvailabilityApi.lookup(...) calls at the same time.
• Return only offers whose availability decision says they can be shown.
• Sort the returned offers by Offer::rank, then Offer::id.
• Use Java stream APIs for the operation.
• Do not use parallelStream() or .parallel().
• Do not fan out unbounded asynchronous work.
• Keep the concurrency limit explicit in code.
• Do not use null as a sentinel for unavailable offers.

Use these nested types:

record Offer(String id, int rank) {}
record Availability(boolean show, String region) {}
static final class AvailabilityApi {
    static Availability lookup(String id) {
        throw new UnsupportedOperationException("provided by production");
    }
}