mapper/batcher: replace connected map with per-node disconnectedAt

The Batcher's connected field (*xsync.Map[types.NodeID, *time.Time])
encoded three states via pointer semantics:

  - nil value:    node is connected
  - non-nil time: node disconnected at that timestamp
  - key missing:  node was never seen

This was error-prone (nil meaning 'connected' inverts Go idioms),
redundant with b.nodes + hasActiveConnections(), and required keeping
two parallel maps in sync. It also contained a bug in RemoveNode where
new(time.Now()) was used instead of &now, producing a zero time.

Replace the separate connected map with a disconnectedAt field on
multiChannelNodeConn (atomic.Pointer[time.Time]), tracked directly
on the object that already manages the node's connections.

Changes:
  - Add disconnectedAt field and helpers (markConnected, markDisconnected,
    isConnected, offlineDuration) to multiChannelNodeConn
  - Remove the connected field from Batcher
  - Simplify IsConnected from two map lookups to one
  - Simplify ConnectedMap and Debug from two-map iteration to one
  - Rewrite cleanupOfflineNodes to scan b.nodes directly
  - Remove the markDisconnectedIfNoConns helper
  - Update all tests and benchmarks

Fixes #3141

hscontrol/mapper/batcher_unit_test.go

@@ -1121,9 +1121,9 @@ func TestBatcher_QueueWorkAfterClose_DoesNotHang(t *testing.T) {
 }
 
 // TestIsConnected_FalseAfterAddNodeFailure is a regression guard for M3.
-// Before the fix, AddNode error paths removed the connection but left
-// b.connected with its previous value (nil = connected). IsConnected
-// would return true for a node with zero active connections.
+// Before the fix, AddNode error paths removed the connection but did not
+// mark the node as disconnected. IsConnected would return true for a
+// node with zero active connections.
 func TestIsConnected_FalseAfterAddNodeFailure(t *testing.T) {
 	b := NewBatcher(50*time.Millisecond, 2, nil)
 	b.Start()
@@ -1132,12 +1132,11 @@ func TestIsConnected_FalseAfterAddNodeFailure(t *testing.T) {
 
 	id := types.NodeID(42)
 
-	// Simulate a previous session leaving the node marked as connected.
-	b.connected.Store(id, nil) // nil = connected
-
 	// Pre-create the node entry so AddNode reuses it, and set up a
 	// multiChannelNodeConn with no mapper so MapResponseFromChange will fail.
+	// markConnected() simulates a previous session leaving it connected.
 	nc := newMultiChannelNodeConn(id, nil)
+	nc.markConnected()
 	b.nodes.Store(id, nc)
 
 	ch := make(chan *tailcfg.MapResponse, 1)