resolver, watcher: a record type whose query fails keeps its previous records (closes #231)

The resolver lists in FailedTypes each record type whose query to a
nameserver got no usable reply (none after two tries, an error reply, a
referral, or a truncated reply whose TCP retry failed), and logs it with
the reason. A nameserver that answered no type has failed, as before.
The watcher saves such a type in failedTypes, keeping the previous
check's records, leaves it out of the comparison with other nameservers
on that check, and compares the kept records with the next answer. With
nothing to keep, it is also in unknownTypes and not compared until it
answers. Change messages leave out what was not compared. A nameserver
whose A, AAAA or CNAME query failed is no answer when following a CNAME
or resolving addresses.

Model: opus-5-5

internal/resolver/export_test.go

@@ -2,10 +2,21 @@ package resolver
 
 import (
 	"context"
+	"log/slog"
+	"time"
 
 	"github.com/miekg/dns"
 )
 
+// NewWithFailingTCP returns a Resolver whose TCP client gives up before
+// it can connect, so the retry over TCP of every truncated reply fails.
+func NewWithFailingTCP(log *slog.Logger) *Resolver {
+	r := NewFromLogger(log)
+	r.tcp = &tcpClient{timeout: time.Nanosecond}
+
+	return r
+}
+
 // ExtractRecordValue exports extractRecordValue for testing.
 func ExtractRecordValue(rr dns.RR) string {
 	return extractRecordValue(rr)