Add Fleet Torrent erasure extension with shard DHT and gossip.

Content-addressed shard DHT on seeder agents with subnet_primary_seeder election, cross-subnet fleet_torrent_gossip, BGP swarm magnets, C2 torrent manifest, and k-of-n peer fetch with C2 fallback.

server/internal/atlas/fleet_gossip.go

@@ -0,0 +1,67 @@
+package atlas
+
+import (
+	"strings"
+)
+
+// Fleet gossip kinds — shard DHT advertisements relayed fleet-wide (not LAN-only).
+const (
+	FleetGossipHaveShard = "have_shard"
+	FleetGossipHealthy   = "healthy"
+	FleetGossipKnowNode  = "know_node"
+)
+
+// FleetGossipRecord is one peer advertisement in the fleet torrent DHT.
+type FleetGossipRecord struct {
+	Kind          string `json:"kind"`
+	AgentID       string `json:"agent_id,omitempty"`
+	Subnet        string `json:"subnet,omitempty"`
+	Token         string `json:"token,omitempty"`
+	ShardIndex    int    `json:"shard_index,omitempty"`
+	ShardHash     string `json:"shard_hash,omitempty"`
+	TargetAgentID string `json:"target_agent_id,omitempty"`
+	Healthy       bool   `json:"healthy,omitempty"`
+	FetchURL      string `json:"fetch_url,omitempty"`
+}
+
+// NormalizeFleetGossipRecord validates and trims one fleet gossip record.
+func NormalizeFleetGossipRecord(r FleetGossipRecord) (FleetGossipRecord, bool) {
+	r.Kind = strings.TrimSpace(strings.ToLower(r.Kind))
+	r.AgentID = strings.TrimSpace(r.AgentID)
+	r.Subnet = strings.TrimSpace(r.Subnet)
+	r.Token = strings.TrimSpace(r.Token)
+	r.ShardHash = strings.TrimSpace(strings.ToLower(r.ShardHash))
+	r.TargetAgentID = strings.TrimSpace(r.TargetAgentID)
+	r.FetchURL = strings.TrimSpace(r.FetchURL)
+	switch r.Kind {
+	case FleetGossipHaveShard:
+		if r.AgentID == "" || r.Token == "" || r.ShardHash == "" {
+			return FleetGossipRecord{}, false
+		}
+	case FleetGossipHealthy:
+		if r.AgentID == "" {
+			return FleetGossipRecord{}, false
+		}
+	case FleetGossipKnowNode:
+		if r.AgentID == "" || r.TargetAgentID == "" {
+			return FleetGossipRecord{}, false
+		}
+	default:
+		return FleetGossipRecord{}, false
+	}
+	return r, true
+}
+
+// NormalizeFleetGossipRecords drops invalid records while preserving order.
+func NormalizeFleetGossipRecords(in []FleetGossipRecord) []FleetGossipRecord {
+	if len(in) == 0 {
+		return nil
+	}
+	out := make([]FleetGossipRecord, 0, len(in))
+	for _, r := range in {
+		if norm, ok := NormalizeFleetGossipRecord(r); ok {
+			out = append(out, norm)
+		}
+	}
+	return out
+}

server/internal/atlas/fleet_gossip_test.go

@@ -0,0 +1,16 @@
+package atlas
+
+import "testing"
+
+func TestNormalizeFleetGossipRecords(t *testing.T) {
+	in := []FleetGossipRecord{
+		{Kind: FleetGossipHaveShard, AgentID: "a", Token: "tok", ShardHash: "abc"},
+		{Kind: "bogus"},
+		{Kind: FleetGossipHealthy, AgentID: "b", Healthy: true},
+		{Kind: FleetGossipKnowNode, AgentID: "a", TargetAgentID: "c"},
+	}
+	out := NormalizeFleetGossipRecords(in)
+	if len(out) != 3 {
+		t.Fatalf("got %d records", len(out))
+	}
+}