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.

testing/sidecar/.env

@@ -7,6 +7,7 @@
 FIPS_NSEC=e752b92aed3ac1595807f5d0eb5125589fbec0a2cfd3a2948d87ea076557deeb
 
 # Peer configuration (leave empty for standalone operation)
-FIPS_PEER_NPUB=
-FIPS_PEER_ADDR=
-FIPS_PEER_ALIAS=peer
+FIPS_PEER_NPUB=npub16xhnhwaxzu3w6dlf88eqnea52cqx9crdwhx4s807zd9nxmng3seqfc587p
+FIPS_PEER_ADDR=217.77.8.91:2121
+FIPS_PEER_ALIAS=vps
+FIPS_PEER_TRANSPORT=udp