feat: cache warming — proactive DNS resolution for configured domains (#78)

Resolves A + AAAA at startup for domains listed in [cache] warm,
then re-resolves before TTL expiry (at 75% elapsed). Keeps critical
domains always hot in cache with zero client-visible latency.

Closes #34 (item 4)

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

src/config.rs

@@ -247,6 +247,8 @@ pub struct CacheConfig {
     pub min_ttl: u32,
     #[serde(default = "default_max_ttl")]
     pub max_ttl: u32,
+    #[serde(default)]
+    pub warm: Vec<String>,
 }
 
 impl Default for CacheConfig {
@@ -255,6 +257,7 @@ impl Default for CacheConfig {
             max_entries: default_max_entries(),
             min_ttl: default_min_ttl(),
             max_ttl: default_max_ttl(),
+            warm: Vec::new(),
         }
     }
 }