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/main.rs

@@ -201,6 +201,7 @@ async fn main() -> numa::Result<()> {
         tls_config: initial_tls,
         upstream_mode: config.upstream.mode,
         root_hints: numa::recursive::parse_root_hints(&config.upstream.root_hints),
+        srtt: std::sync::RwLock::new(numa::srtt::SrttCache::new(config.upstream.srtt)),
         dnssec_enabled: config.dnssec.enabled,
         dnssec_strict: config.dnssec.strict,
     });
@@ -353,8 +354,13 @@ async fn main() -> numa::Result<()> {
         let prime_ctx = Arc::clone(&ctx);
         let prime_tlds = config.upstream.prime_tlds;
         tokio::spawn(async move {
-            numa::recursive::prime_tld_cache(&prime_ctx.cache, &prime_ctx.root_hints, &prime_tlds)
-                .await;
+            numa::recursive::prime_tld_cache(
+                &prime_ctx.cache,
+                &prime_ctx.root_hints,
+                &prime_tlds,
+                &prime_ctx.srtt,
+            )
+            .await;
         });
     }