Implement comprehensive node and transport statistics

Add 71 new counters (84 values) across three categories:

Node statistics (NodeStats, plain u64 — single handler context):
- Forwarding: 9 counters x (packets + bytes) = 18 values. Covers
  received, decode_error, ttl_exhausted, delivered, forwarded,
  drop_no_route, drop_mtu_exceeded, drop_send_error, originated.
- Discovery: 17 counters (packets only). Request path: received,
  decode_error, duplicate, already_visited, target_is_us, forwarded,
  ttl_exhausted, initiated, deduplicated. Response path: received,
  decode_error, forwarded, identity_miss, proof_failed, accepted,
  timed_out.
- Error signals: 3 counters — coords_required, path_broken,
  mtu_exceeded.
- Spanning tree: 16 counters. Inbound announce handling (received
  through accepted, parent switch, loop detection, ancestry change),
  outbound (sent, rate limited, send failed), cumulative events
  (parent switches/losses, flap dampening).
- Bloom filter: 10 counters. Inbound (received through accepted),
  outbound (sent, debounce suppressed, send failed).

Transport statistics (AtomicU64 + Arc — shared with spawned tasks):
- UDP (6 counters, 8 values): packets/bytes sent/recv, send_errors,
  recv_errors, mtu_exceeded, kernel_drops (stub for SO_MEMINFO).
- TCP (10 counters, 12 values): packets/bytes sent/recv, send_errors,
  recv_errors, mtu_exceeded, plus connection lifecycle counters
  (established, accepted, rejected, timeouts, refused).

Control socket integration:
- show_routing: forwarding, discovery, error signal stats
- show_tree: spanning tree stats + per-peer bloom metrics
  (estimated_count, set_bits, fill_ratio) and coordinate paths
- show_bloom: bloom filter stats + per-peer snapshots
- show_transports: per-transport stats snapshots

Also refactor UDP transport from flat files (udp.rs + udp_stats.rs)
into directory module (udp/mod.rs + udp/stats.rs) matching TCP
structure, and fix pre-existing clippy warnings in tree/tests.rs.

src/node/mod.rs

@@ -13,6 +13,7 @@ mod routing_error_rate_limit;
 pub(crate) mod session;
 pub(crate) mod session_wire;
 pub(crate) mod wire;
+pub(crate) mod stats;
 mod tree;
 #[cfg(test)]
 mod tests;
@@ -299,6 +300,10 @@ pub struct Node {
     /// Next transport ID to allocate.
     next_transport_id: u32,
 
+    // === Node Statistics ===
+    /// Routing, forwarding, discovery, and error signal counters.
+    stats: stats::NodeStats,
+
     // === TUN Interface ===
     /// TUN device state.
     tun_state: TunState,
@@ -433,6 +438,7 @@ impl Node {
             max_links,
             next_link_id: 1,
             next_transport_id: 1,
+            stats: stats::NodeStats::new(),
             tun_state,
             tun_name: None,
             tun_tx: None,
@@ -526,6 +532,7 @@ impl Node {
             max_links,
             next_link_id: 1,
             next_transport_id: 1,
+            stats: stats::NodeStats::new(),
             tun_state,
             tun_name: None,
             tun_tx: None,
@@ -803,6 +810,18 @@ impl Node {
         &mut self.coord_cache
     }
 
+    // === Node Statistics ===
+
+    /// Get the node statistics.
+    pub fn stats(&self) -> &stats::NodeStats {
+        &self.stats
+    }
+
+    /// Get mutable node statistics.
+    pub(crate) fn stats_mut(&mut self) -> &mut stats::NodeStats {
+        &mut self.stats
+    }
+
     // === TUN Interface ===
 
     /// Get the TUN state.