fix(artery): distribute ActorSelection messages across outbound lanes by target path

[He-Pin]

Motivation

The "must be able to send messages with actorSelection concurrently preserving order" test flakes on CI: 4 sender actors each drive 1000 round-trips via ActorSelection (4004 messages total). With multi-lane artery config (outbound-lanes > 1), all ActorSelection messages are routed to the same outbound queue because selectQueue uses the anchor's UID as the distribution key:

OrdinaryQueueIndex + (math.abs(r.path.uid % outboundLanes))

The anchor for ActorSelection is the root guardian (RootActorPath), whose UID is always 0 (ActorCell.undefinedUid). So math.abs(0 % N) = 0 for any N — all ActorSelection traffic concentrates on lane 0 while other lanes sit idle.

In contrast, the ActorRef variant distributes across lanes because each echo actor has a distinct non-zero UID.

This is not a recent regression — the selectQueue logic has been unchanged since the Pekko fork from Akka. PR #3090 (timeout widening) masks the symptom but doesn't fix the structural bottleneck.

Modification

Add a dedicated case sel: ActorSelectionMessage (non-PriorityMessage) in Association.send that computes the queue index from the selection's target path elements hash instead of the anchor's UID:

case sel: ActorSelectionMessage =>
  val queueIndex =
    if (outboundLanes == 1) OrdinaryQueueIndex
    else OrdinaryQueueIndex + (math.abs(sel.elements.hashCode()) % outboundLanes)
  val queue = queues(queueIndex)
  if (!queue.offer(outboundEnvelope))
    dropped(queueIndex, queueSize, outboundEnvelope)

This distributes ActorSelection messages across lanes by their target path while preserving per-path message ordering (same target path → same hash → same lane).

PriorityMessage ActorSelection (used by cluster heartbeats) continues to go through the control queue unchanged.

Result

ActorSelection messages are now distributed across all outbound lanes based on target path, eliminating the single-lane throughput bottleneck. The existing ActorRef-based test and PriorityMessage routing are unaffected.

Tests

• sbt "remote / Test / compile" — compile check
• CI will exercise the artery variants (1-lane and 3-lane configs)

References

Refs #3041 (previous timeout widening), supersedes #3090

[pjfanning]

#3089 test is failing in this PR - we appear to have to broken this with some change over the last week or 2. This test is now very flaky and I don't recall it being an issue before

[pjfanning]

#3089 is happening for tests that don't use actor selection but maybe not quite as often.