refactor: extract resolve_coalesced, test real code (#21)

* refactor: extract resolve_coalesced, rewrite tests against real code

Extract Disposition enum, acquire_inflight(), and resolve_coalesced()
from handle_query so coalescing logic is independently testable. Rewrite
integration tests to call resolve_coalesced directly with mock futures
instead of fighting the iterative resolver's NS chain. All 12 coalescing
tests now exercise production code paths, not tokio primitives.

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

* fix: SERVFAIL echoes question section, preserve error messages

resolve_coalesced now takes &DnsPacket instead of query_id so SERVFAIL
responses use response_from (echoing question section per RFC). Error
messages preserved via Option<String> return for upstream error logging.

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

---------

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

src/stats.rs

@@ -13,7 +13,7 @@ pub struct ServerStats {
     started_at: Instant,
 }
 
-#[derive(Clone, Copy, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum QueryPath {
     Local,
     Cached,