Add a generic BackpressureQueue that uses a DropPredicate to proactively
shed load before the queue is completely full.
Two predicate types are provided:
- DropCheckFunc: length-only drop decision (func(queueLen int) bool)
- DropPredicate[T]: item-aware drop decision
RandomEarlyDrop returns a DropCheckFunc since RED only considers queue
depth. The AsDropPredicate helper adapts it to DropPredicate[T] for use
with BackpressureQueue.
In addition to the blocking Enqueue/Dequeue methods, the queue exposes
TryEnqueue (non-blocking send with drop check), Len, ReceiveChan, and
Close. These are needed by the actor package's BackpressureMailbox which
uses BackpressureQueue as its core buffer while implementing the Mailbox
interface's select-based iteration and lifecycle methods.
Property-based tests using pgregory.net/rapid verify queue invariants
(capacity bounds, FIFO ordering, model consistency) across randomized
enqueue/dequeue sequences with RED enabled.
