Zero the vacated head slot in concurrent.Queue.Dequeue to release dead references - #598

What

Queue.Dequeue advanced the internal slice with q.items = q.items[1:] but never cleared the old head slot. The backing array therefore kept a live pointer to the dequeued value at the vacated index until the array eventually reallocated.

This change zeroes the head slot before re-slicing and drops the backing array once the queue empties:

item := q.items[0]
var zero T
q.items[0] = zero
q.items = q.items[1:]
if len(q.items) == 0 {
    q.items = nil
}
return item, true

Why

The queue is instantiated with reference element types that are long-lived and cycle steadily, e.g. Queue[*execution.MessageEnvelope] (workflow/inproc/runner.go), Queue[workflow.Event] (workflow/internal/execution/eventstream.go), and Queue[func(context.Context) error] (workflow/inproc/context.go). Because the head slot was never cleared, a dequeued value stayed GC-reachable through the backing array long after it was logically removed, retaining whatever object graph it pointed at until the slice happened to reallocate.

Clearing the vacated element mirrors Go's own slices.Delete semantics, which zero removed slots precisely so dead references can be collected promptly.

How it is tested

Two deterministic tests in internal/concurrent/queue_test.go:

• TestQueue_DequeueZeroesVacatedSlot inspects the shared backing array through a full-capacity view and asserts the head slot is nil after Dequeue (while later elements keep the array alive).
• TestQueue_DequeueDropsEmptyBackingArray asserts items is released to nil once the queue drains.

Both fail before the change and pass after. go build ./..., go vet ./internal/concurrent/..., and go test ./internal/concurrent/... all pass.