perf: optimize DNS query hot path (#15)

* perf: optimize hot path — RwLock, inline filtering, pre-allocated strings

- Mutex → RwLock for cache, blocklist, and overrides (concurrent read access)
- Make cache.lookup() and overrides.lookup() take &self (read-only)
- Eliminate 3 Vec allocations per DnsPacket::write() via inline filtering
- Pre-allocate domain strings with capacity 64 in parse path
- Add criterion micro-benchmarks (hot_path + throughput)
- Add bench README documenting both benchmark suites

Measured improvement: ~14% faster parsing, ~9% pipeline throughput,
round-trip cached 733ns → 698ns (~2.3M queries/sec).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: simplify benchmark code after review

- Remove redundant DnsHeader::new() (already set by DnsPacket::new())
- Remove unused DnsHeader import
- Change simulate_cached_pipeline to take &DnsCache (lookup is &self now)
- Remove unnecessary mut on cache in cache_lookup_miss bench

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -28,3 +28,14 @@ time = "0.3"
 rustls = "0.23"
 tokio-rustls = "0.26"
 arc-swap = "1"
+
+[dev-dependencies]
+criterion = { version = "0.5", features = ["html_reports"] }
+
+[[bench]]
+name = "hot_path"
+harness = false
+
+[[bench]]
+name = "throughput"
+harness = false