feat: live mesh peer gossip

When a peer's hello is verified, send a gossip burst of currently
connected peers so the new peer can discover and connect to nodes it
hasn't met yet — without relying on the anchor for a full peer-join
notification for each one.

- mesh: add PendingConnect chan for gossip-discovered peer IDs
- peer: send gossip burst after hello; push unknown peers from incoming
  gossip onto PendingConnect (skip self + already-connected)
- anchor: drain PendingConnect per runOnce, initiate startOffer for
  unknown peers using the same lexicographic tiebreak as anchor join;
  drain goroutine is scoped to the runOnce lifetime to avoid using stale
  sessions after a reconnect

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mesh/mesh.go

@@ -38,6 +38,10 @@ type Mesh struct {
 	outbound   map[string]*outboundTransfer // xid → pending outbound
 	inbound    map[string]*inboundTransfer  // xid → pending inbound
 
+	// PendingConnect receives peer IDs discovered via gossip that we should
+	// attempt to connect to. Drained by the anchor client's runOnce loop.
+	PendingConnect chan proto.PeerID
+
 	// subscribers receive a copy of every event (fan-out to IPC clients)
 	subMu sync.Mutex
 	subs  []chan proto.IpcMessage
@@ -47,11 +51,12 @@ type Mesh struct {
 // Pass a non-nil store to enable message and peer persistence.
 func New(id *crypto.Identity, st *store.Store) *Mesh {
 	return &Mesh{
-		Identity: id,
-		Store:    st,
-		peers:    make(map[proto.PeerID]*PeerConn),
-		outbound: make(map[string]*outboundTransfer),
-		inbound:  make(map[string]*inboundTransfer),
+		Identity:       id,
+		Store:          st,
+		peers:          make(map[proto.PeerID]*PeerConn),
+		outbound:       make(map[string]*outboundTransfer),
+		inbound:        make(map[string]*inboundTransfer),
+		PendingConnect: make(chan proto.PeerID, 32),
 	}
 }