feat: wire-level forwarding, cache, request hedging, and DoH keepalive

Wire-level forwarding path skips DnsPacket parse/serialize on the hot
path. Cache stores raw wire bytes with pre-scanned TTL offsets — patches
ID + TTLs in-place on lookup instead of cloning parsed packets.

Request hedging (Dean & Barroso "Tail at Scale") fires a second
parallel request after a configurable delay (default 10ms) when
the primary upstream stalls. DoH keepalive loop prevents idle
HTTP/2 + TLS connection teardown.

Recursive resolver now hedges across multiple NS addresses and
caches NS delegation records to skip TLD re-queries.

Integration test harness polls /blocking/stats instead of fixed
sleep, eliminating the blocklist-download race condition.

src/lib.rs

@@ -26,6 +26,7 @@ pub mod srtt;
 pub mod stats;
 pub mod system_dns;
 pub mod tls;
+pub mod wire;
 
 pub type Error = Box<dyn std::error::Error + Send + Sync>;
 pub type Result<T> = std::result::Result<T, Error>;