feat: add COALESCED query path and coalescing tests

Followers in the inflight coalescing path now log as COALESCED instead
of RECURSIVE, making it visible in the dashboard when queries were
deduplicated vs independently resolved. Adds 10 tests covering
InflightGuard cleanup, broadcast mechanics, and concurrent handle_query
coalescing through a mock TCP DNS server.

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

site/dashboard.html

@@ -285,6 +285,7 @@ body {
 .path-tag.OVERRIDE { background: rgba(82, 122, 82, 0.12); color: var(--emerald); }
 .path-tag.SERVFAIL { background: rgba(181, 68, 58, 0.12); color: var(--rose); }
 .path-tag.BLOCKED { background: rgba(163, 152, 136, 0.15); color: var(--text-dim); }
+.path-tag.COALESCED { background: rgba(138, 104, 158, 0.12); color: var(--violet-dim); }
 
 /* Sidebar panels */
 .sidebar {
@@ -547,6 +548,8 @@ body {
           <select id="logFilterPath" onchange="applyLogFilter()"
             style="font-family:var(--font-mono);font-size:0.7rem;padding:0.25rem 0.4rem;border:1px solid var(--border);border-radius:4px;background:var(--bg-surface);color:var(--text-secondary);outline:none;">
             <option value="">all paths</option>
+            <option value="RECURSIVE">recursive</option>
+            <option value="COALESCED">coalesced</option>
             <option value="FORWARD">forward</option>
             <option value="CACHED">cached</option>
             <option value="BLOCKED">blocked</option>