Implement comprehensive node and transport statistics

[jmcorgan]

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.