watcher: notify NS query failure and recovery (closes #104)

LookupAllRecords now returns each nameserver's response, so the
watcher saves its status: ok when it answered, NXDOMAIN and no records
included, and error with the reason when it timed out, answered
SERVFAIL or REFUSED, or could not be reached. A nameserver that starts
failing sends NS Failure and one that answers again sends NS Recovery.
A failing nameserver is left out of the record change and
inconsistency comparisons. The resolver used to report REFUSED and
network errors as an answer with no records; they are now errors. A
lookup cut short by its context now returns an error instead of a
failure of the nameserver it was querying.

Model: opus-5-5

internal/watcher/inconsistency_test.go

@@ -105,7 +105,9 @@ func TestNewlyDisagreeingPairs(t *testing.T) {
 
 			prev := hostnameState(tt.loaded)
 
-			for i, current := range tt.checks {
+			for i, records := range tt.checks {
+				current := hostnameState(records)
+
 				got := watcher.NewlyDisagreeingPairs(prev, current)
 				if !slices.Equal(got, tt.want[i]) {
 					t.Errorf(
@@ -114,7 +116,7 @@ func TestNewlyDisagreeingPairs(t *testing.T) {
 					)
 				}
 
-				prev = hostnameState(current)
+				prev = current
 			}
 		})
 	}
@@ -162,8 +164,9 @@ func TestInconsistencyAlert(t *testing.T) {
 			prev := hostnameState(tt.loaded)
 
 			for range 3 {
-				w.DetectHostnameChanges(t.Context(), host, prev, disagree)
-				prev = hostnameState(disagree)
+				current := hostnameState(disagree)
+				w.DetectHostnameChanges(t.Context(), host, prev, current)
+				prev = current
 			}
 
 			got := 0