feat: swarm membership discovery via peer-list gossip

Refs #1

- /v1/ping now advertises the node's known peers (peer_id, addr, name)
- Run sweeps static KNOX_PEERS + persisted discovered peers, enqueueing
  newly-learned addresses for direct sweeps (membership-only relay; no
  observation relay)
- db: ShareablePeers, SwarmPeerAddrs, MergePeer (cursor-preserving
  discovery upsert), MaxHCLForNode
- integration test: a node configured with a single seed discovers and
  pulls from other swarm members without direct configuration

internal/watch/gossip_test.go

@@ -84,8 +84,77 @@ func TestGossipConvergence(t *testing.T) {
 	}
 }
 
-// TestGossipDiff ensures /v1/diff reports per-node observation fingerprints and
-// tombstoned thread divergence.
+// TestGossipSwarmDiscovery: C only knows A. A knows B. When C sweeps A, it must
+// learn about B through A's ping, enqueue B, and pull B's observations — with no
+// direct configuration of B (no relay of data, only membership).
+func TestGossipSwarmDiscovery(t *testing.T) {
+	a := tmpKnoxDB(t)
+	b := tmpKnoxDB(t)
+	c := tmpKnoxDB(t)
+
+	seedObs(a, "AAA")
+	seedObs(b, "BBB")
+	seedObs(c, "CCC")
+
+	nodeA := &Node{Kdb: a, Name: "A"}
+	sa := httptest.NewServer(nodeA.Handler())
+	defer sa.Close()
+	nodeB := &Node{Kdb: b, Name: "B"}
+	sb := httptest.NewServer(nodeB.Handler())
+	defer sb.Close()
+	nodeC := &Node{Kdb: c, Name: "C"}
+	sc := httptest.NewServer(nodeC.Handler())
+	defer sc.Close()
+
+	// A discovers B (A pings B) so A can advertise B to the swarm.
+	Run(a, []string{sb.URL})
+
+	// C only knows A. A single sweep should surface B (membership in ping)
+	// and pull B's observations directly.
+	Run(c, []string{sa.URL})
+
+	// C must know B and hold all three origin logs.
+	peers, err := c.ListPeers()
+	if err != nil {
+		t.Fatalf("list peers: %v", err)
+	}
+	foundB := false
+	for _, p := range peers {
+		if p.PeerID == b.NodeID() {
+			foundB = true
+		}
+	}
+	if !foundB {
+		t.Fatalf("C did not discover B via A's membership list; peers=%v", peers)
+	}
+
+	vec, err := c.KnowledgeVector()
+	if err != nil {
+		t.Fatalf("c vector: %v", err)
+	}
+	if len(vec) != 3 {
+		t.Errorf("C should hold 3 origin logs (A, B, C), got %v", vec)
+	}
+
+	// C's copy of B's log must match B's own max hcl.
+	bmax := c.MaxHCLForNode(b.NodeID())
+	bm, err := bNodeMax(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if bmax != bm {
+		t.Errorf("C max hcl for B=%d, B reports %d", bmax, bm)
+	}
+}
+
+// bNodeMax is a helper reading the max hcl B holds for its own node.
+func bNodeMax(b *db.KnoxDB) (int64, error) {
+	rows, err := b.KnowledgeVector()
+	if err != nil {
+		return 0, err
+	}
+	return rows[b.NodeID()], nil
+}
 func TestGossipDiff(t *testing.T) {
 	a := tmpKnoxDB(t)
 	b := tmpKnoxDB(t)