fix(model): TXT сравнивается байт-точно (без схлопывания пробелов)

internal/model/record.go

@@ -54,10 +54,11 @@ func normalizeName(name string) string {
 
 // normalizeValue canonicalizes a single RR value for comparison.
 func normalizeValue(t RecordType, content string) string {
+	if t == TXT {
+		return content // byte-exact — case and whitespace are significant (DKIM/SPF/DMARC)
+	}
 	c := strings.Join(strings.Fields(content), " ") // collapse whitespace
 	switch t {
-	case TXT:
-		return c // case-sensitive — keep as is
 	case MX:
 		parts := strings.SplitN(c, " ", 2)
 		if len(parts) == 2 {

internal/model/record_test.go

@@ -99,6 +99,14 @@ func TestEqualTXTCaseSensitive(t *testing.T) {
 	}
 }
 
+func TestEqualTXTWhitespaceSignificant(t *testing.T) {
+	a := Record{Type: TXT, Name: "example.com.", TTL: 60, Values: []string{"v=spf1  a"}}
+	b := Record{Type: TXT, Name: "example.com.", TTL: 60, Values: []string{"v=spf1 a"}}
+	if a.Equal(b) {
+		t.Fatal("TXT records differing only in whitespace count must not be equal (byte-exact comparison)")
+	}
+}
+
 func TestEqualTTLMatters(t *testing.T) {
 	a := Record{Type: A, Name: "example.com.", TTL: 300, Values: []string{"1.2.3.4"}}
 	b := Record{Type: A, Name: "example.com.", TTL: 600, Values: []string{"1.2.3.4"}}