client/conn_state_pusher: nil-receiver safe On* entry points

Fixes a panic on Engine.Stop teardown:

  Engine.Stop()
    e.connStatePusher.Stop()
    e.connStatePusher = nil          ← nilled here
    ...
    e.removeAllPeers()                ← still triggers
      Conn.Close
        UpdatePeerState               ← fires status listener
          listener-closure
            e.connStatePusher.        ← nil receiver
              OnPeerStateChange()     ← deref panic at p.events <- ev

Test_Engine_SSH on the netbirdio CI runners hit this reliably because
the runners can actually create the wg interface (sandbox cannot, so
the test fails earlier locally and never reaches the Stop path).

Stack: client/internal/conn_state_pusher.go:99 → engine.go:633 →
peer/status.go:300 (notify closure) → peer/conn.go:736 → conn.go:329
→ conn_mgr.go:454 → engine.go:864 → engine.go:852 → engine.go:391.

Defending in the pusher itself (vs. nil-checking each callsite) keeps
the engine cleanup path simple and matches the no-op-on-nil pattern
already used elsewhere in the package.

Author: MichaelUray

client/internal/conn_state_pusher.go

@@ -94,7 +94,16 @@ func (p *connStatePusher) Stop() {
 
 // OnPeerStateChange enqueues a state-change event. Non-blocking — drops
 // if the buffer is full (the next bulk tick will catch up via delta).
+//
+// Safe on a nil receiver: Engine.Stop nils e.connStatePusher before
+// removeAllPeers runs, but the status-recorder listener registered in
+// Engine.Start is still wired and may fire a few more events during
+// peer cleanup. A nil-receiver no-op makes the cleanup path cheap and
+// avoids a panic on the engine shutdown race.
 func (p *connStatePusher) OnPeerStateChange(ev PeerStateChangeEvent) {
+	if p == nil {
+		return
+	}
 	select {
 	case p.events <- ev:
 	default:
@@ -103,8 +112,12 @@ func (p *connStatePusher) OnPeerStateChange(ev PeerStateChangeEvent) {
 
 // OnSnapshotRequest enqueues a snapshot-request nonce. Non-blocking,
 // coalescing — multiple requests in flight result in a single full
-// snapshot with the latest nonce echoed.
+// snapshot with the latest nonce echoed. Nil-receiver safe for the
+// same shutdown-race reason as OnPeerStateChange.
 func (p *connStatePusher) OnSnapshotRequest(nonce uint64) {
+	if p == nil {
+		return
+	}
 	select {
 	case p.snapshotReq <- nonce:
 	default: