feat: SRTT-based nameserver selection (#19)

* feat: SRTT-based nameserver selection for recursive resolver

BIND-style Smoothed RTT (EWMA) tracking per NS IP address. The resolver
learns which nameservers respond fastest and prefers them, eliminating
cascading timeouts from slow/unreachable IPv6 servers.

- New src/srtt.rs: SrttCache with record_rtt, record_failure, sort_by_rtt
- EWMA formula: new = (old * 7 + sample) / 8, 5s failure penalty, 5min decay
- TCP penalty (+100ms) lets SRTT naturally deprioritize IPv6-over-TCP
- Enabled flag embedded in SrttCache (no-op when disabled)
- Batch eviction (64 entries) for O(1) amortized writes at capacity
- Configurable via [upstream] srtt = true/false (default: true)
- Benchmark script: scripts/benchmark.sh (full, cold, warm, compare-all)
- Benchmarks show 12x avg improvement, 0% queries >1s (was 58%)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: show DNSSEC and SRTT status in dashboard + API

Add dnssec and srtt boolean fields to /stats API response.
Display on/off indicators in the dashboard footer.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: apply SRTT decay before EWMA so recovered servers rehabilitate

Without decay-before-EWMA, a server penalized at 5000ms stayed near
that value even after recovery — the stale raw penalty was used as the
EWMA base instead of the decayed estimate. Extract decayed_srtt()
helper and call it in record_rtt() before the smoothing step.

Also restores removed "why" comments in send_query / resolve_recursive.

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

* docs: add install/upgrade instructions, smarter benchmark priming

README: document `numa install`, `numa service`, Homebrew upgrade,
and `make deploy` workflows. Benchmark: replace fixed `sleep 4` with
`wait_for_priming` that polls cache entry count for stability.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/config.rs

@@ -85,6 +85,8 @@ pub struct UpstreamConfig {
     pub root_hints: Vec<String>,
     #[serde(default = "default_prime_tlds")]
     pub prime_tlds: Vec<String>,
+    #[serde(default = "default_srtt")]
+    pub srtt: bool,
 }
 
 impl Default for UpstreamConfig {
@@ -96,10 +98,15 @@ impl Default for UpstreamConfig {
             timeout_ms: default_timeout_ms(),
             root_hints: default_root_hints(),
             prime_tlds: default_prime_tlds(),
+            srtt: default_srtt(),
         }
     }
 }
 
+fn default_srtt() -> bool {
+    true
+}
+
 fn default_prime_tlds() -> Vec<String> {
     vec![
         // gTLDs