Resilience4j @Bulkhead(maxConcurrentCalls=2, maxWaitDuration=100ms), same 4-caller/300ms shape
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REJECTED:c0 in 112ms
done:c1 in 307ms
done:c2 in 303ms
REJECTED:c3 in 101ms

REJECTED count: 2 -- these callers waited up to maxWaitDuration=100ms for a slot,
then gave up and ran the fallback method, instead of blocking indefinitely like @ConcurrencyLimit's BLOCK policy.
