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>

src/record.rs

@@ -70,7 +70,7 @@ impl DnsRecord {
     }
 
     pub fn read(buffer: &mut BytePacketBuffer) -> Result<DnsRecord> {
-        let mut domain = String::new();
+        let mut domain = String::with_capacity(64);
         buffer.read_qname(&mut domain)?;
 
         let qtype_num = buffer.read_u16()?;
@@ -110,7 +110,7 @@ impl DnsRecord {
                 Ok(DnsRecord::AAAA { domain, addr, ttl })
             }
             QueryType::NS => {
-                let mut ns = String::new();
+                let mut ns = String::with_capacity(64);
                 buffer.read_qname(&mut ns)?;
 
                 Ok(DnsRecord::NS {
@@ -120,7 +120,7 @@ impl DnsRecord {
                 })
             }
             QueryType::CNAME => {
-                let mut cname = String::new();
+                let mut cname = String::with_capacity(64);
                 buffer.read_qname(&mut cname)?;
 
                 Ok(DnsRecord::CNAME {
@@ -131,7 +131,7 @@ impl DnsRecord {
             }
             QueryType::MX => {
                 let priority = buffer.read_u16()?;
-                let mut mx = String::new();
+                let mut mx = String::with_capacity(64);
                 buffer.read_qname(&mut mx)?;
 
                 Ok(DnsRecord::MX {