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.

Cargo.toml

@@ -37,6 +37,8 @@ webpki-roots = "1"
 criterion = { version = "0.8", features = ["html_reports"] }
 tower = { version = "0.5", features = ["util"] }
 http = "1"
+hickory-resolver = { version = "0.25", features = ["https-ring", "webpki-roots"] }
+hickory-proto = "0.25"
 
 [[bench]]
 name = "hot_path"
@@ -49,3 +51,7 @@ harness = false
 [[bench]]
 name = "dnssec"
 harness = false
+
+[[bench]]
+name = "recursive_compare"
+harness = false