docs: interconnect troubleshooting runbook (diagnosing tier1 peers via pg_cluster_ic_peers)

[sqlrush]

Summary

We have a complete reference for pg_cluster_ic_peers
(docs/reference/system-views.md) and a working multi-node walkthrough
(docs/user-guide/bootstrap.md), but no task-oriented troubleshooting guide
for when the tier1 (TCP) interconnect doesn't come up. This issue adds one.

Why this is a good first issue

• Pure documentation — no C, no cluster internals.
• Everything you need is already in the tree: the pg_cluster_ic_peers column
  reference and the cluster.interconnect_* GUC docs.
• You can reproduce every failure state locally with two nodes on loopback
  (see the multi-node section of bootstrap.md).

What to do

Add docs/user-guide/troubleshooting-interconnect.md and link it from
bootstrap.md. For each common failure give the symptom as seen in
pg_cluster_ic_peers, the likely cause, and the fix. At minimum cover:

1. pg_cluster_ic_peers returns zero rows → cluster.interconnect_tier is
   stub, not tier1.
2. Peer stuck state = down, connect_error_count climbing, last_error = "connect SO_ERROR: Connection refused" (errno 61), last_error_code = 08001
   → peer not started, or wrong interconnect_addr / port unreachable.
3. Peer flapping → non-zero reconnect_count; how to read last_connect_at
   vs last_recv_at.
4. Heartbeats not advancing → compare heartbeat_send_count /
   heartbeat_recv_count across two queries a few seconds apart.
5. state = rejected → what rejects a peer (membership/handshake) and where to
   look next.

Tip: to reproduce (2), start only node 0 from the bootstrap walkthrough and
watch node 0's view of node 1 before node 1 is up.

Definition of done

• New docs/user-guide/troubleshooting-interconnect.md, each scenario as
  symptom → cause → fix.
• Linked from docs/user-guide/bootstrap.md.
• Column/GUC names match the existing docs (no invented fields).

Pointers

• docs/reference/system-views.md → ## pg_cluster_ic_peers
• docs/user-guide/configuration.md → cluster.interconnect_*
• docs/user-guide/bootstrap.md → "Multi-node cluster (tier1 TCP interconnect)"