feat(cluster): RFC 010 c10–c16, follow-ups and Phase 6 (squash of 16ef583..d9c62a8)

Tree snapshot of d9c62a8 (2026-08-18). The 20 source commits between
16ef583 (c9) and d9c62a8 were never pushed and the clone that held them
was lost; this commit carries their combined tree verbatim so the build
history stays auditable from the c1–c9 commits below it. Original
hashes as recorded in the session handoff:

  c10  f03e94d  pid targeting + auto-serialization (RemotePid, D14 name
                on the wire); Phase 3 gate
  c11  7ef4bad  DownReason::Disconnected, wire tag 5
  c12  d124162  remote monitors (Monitor/Demonitor/Down frames)
  c13  9de967b  connection-loss synthesis (A+B: Monitors::teardown +
                unread-command Disconnected); Phase 4 gate
  c14  7e822b7  eager pg eviction (reaper actor, ReaperInboxes)
       dbe1a22  InboundVerdict::label(), trace::Event::ClusterInbound
       31a9877  tests/channel.rs monitor-churn target gated on `go`
       653559e  Discovery::Withdrawn{name, addr}
  c15  b41d76e  distributed pg: Sync on NodeUp, Join/Leave broadcast,
                NodeDown sweep, members_all; PgMsg wire type
  c16  fafa881  pick_any / dispatch_any; Phase 5 complete
  Phase 6 Tier A:
       195c73e  p4  NodeEvent::NodeDown(NodeInfo)
       48fd766  p1  connector Candidate{name, addr, state}
       ce8cf99  p2+p7 conn.rs select arms as Vec<Arm>; Outbound::Drained
       bf24988  p6  RemotePid::from_local -> Option
       9ae0380  p3  PeerStanding{Free, Claimed, Dialing}
       c7d62a1  p11 cluster::Timing knobs, threaded by value
       46f171d  p11 cluster_disconnect un-ignored on SMARM_FAST_TIMING
  Phase 6 Tier B:
       391a9ae  p5  cluster::RemoteDownReason{Local, Disconnected};
                    DownReason::Disconnected removed from core
       7ddd908  p9  pg ctl channel unconditional, one cfg seam at spawn
       d9c62a8      PeerNameMismatch parks the candidate; ClusterDial trace

Verified at d9c62a8: default 361/0, cluster 448/0, clippy --lib on
default / cluster / cluster+smarm-trace, fmt, 10x flake on
cluster_dial_mismatch, 5x on cluster_pg.

tests/cluster_envelope.rs

@@ -8,6 +8,7 @@ use smarm::cluster::envelope::{
     decode_payload, encode_payload, DecodeError, Frame, NodeMeta, RejectReason, MAX_FRAME_LEN,
     PROTO_VERSION,
 };
+use smarm::cluster::RemoteDownReason;
 use smarm::monitor::DownReason;
 use smarm::pg::Incarnation;
 
@@ -55,7 +56,11 @@ fn all_frames() -> Vec<Frame> {
         Frame::Demonitor { monitor_id: 77 },
         Frame::Down {
             monitor_id: 77,
-            reason: DownReason::Panic,
+            reason: RemoteDownReason::Local(DownReason::Panic),
+        },
+        Frame::Down {
+            monitor_id: 78,
+            reason: RemoteDownReason::Disconnected,
         },
     ]
 }
@@ -151,7 +156,7 @@ fn unknown_enum_tags() {
     assert_eq!(
         Frame::decode(&buf),
         Err(DecodeError::UnknownEnumTag {
-            what: "DownReason",
+            what: "RemoteDownReason",
             tag: 200
         })
     );