node: classify transit forwards by route class; regroup fipstop routing tab

Add six forwarding counters that partition transit-forwarded packets by their
tree relationship to the chosen next hop: tree-up (peer is our ancestor),
tree-down (peer is our descendant and the destination is within its subtree),
tree-down-cross (peer is our descendant but the destination is outside its
subtree), cross-link descend (lateral peer, destination within its subtree),
cross-link ascend (lateral peer, destination outside its subtree), and
direct-peer. The six classes sum to forwarded_packets, asserted by a unit
test. Classification is computed from tree coordinates at the transit
chokepoint, so the error-signal routing callers are excluded.

The two "outside the chosen peer's subtree" classes are both up-and-over
forwards but differ in what they depend on. Tree-down-cross is the
dive-to-tree-child cut-through: we forward down to our own child for a
destination not beneath it, which is only possible because the child
advertised cross-link reach upward to us, beyond its own subtree. Its count
measures how much forwarding depends on that upward advertisement, i.e. what
would change if cross-link advertisements were narrowed to subtree-entry only.
Cross-link ascend, by contrast, uses the node's own lateral cross-link learned
from a peer's split-horizon advertisement, so it does not depend on any upward
advertisement.

Surface the counters through the forwarding stats snapshot (control socket,
show_routing and show_status) and reorganize the fipstop routing tab so its
two columns separate own/endpoint traffic (received, delivered, originated)
from forwarded/transit traffic (the route-class breakdown and drop reasons),
with the tree-down-cross line visually flagged.

src/node/stats.rs

@@ -229,6 +229,12 @@ pub struct ForwardingStatsSnapshot {
     pub drop_send_error_bytes: u64,
     pub originated_packets: u64,
     pub originated_bytes: u64,
+    pub route_tree_up: u64,
+    pub route_tree_down: u64,
+    pub route_tree_down_cross: u64,
+    pub route_crosslink_descend: u64,
+    pub route_crosslink_ascend: u64,
+    pub route_direct_peer: u64,
 }
 
 #[derive(Clone, Debug, Default, Serialize)]