Refuse [::], 0.0.0.0, IPv6 multicast and documentation space (closes #341)

On Linux a connection to the unspecified address [::] or 0.0.0.0
reaches the host's own loopback, and the SSRF guard let [::] through.
Both unspecified addresses now sit in alwaysBlockedNetworks, so an
allowlist reaches loopback only by naming it; ::/128 joins the default
blocklist beside 0.0.0.0/8. IPv6 multicast (ff00::/8) and documentation
space (2001:db8::/32) are refused by default. Every default blocklist
entry gets a one-line comment, and the README, the rules above each
list and the two pinning tests follow.

Model: opus-5-5

internal/delivery/ssrf_test.go

@@ -101,6 +101,42 @@ func TestValidateTargetURL_Blocked(t *testing.T) {
 	}
 }
 
+// TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation
+// covers the unspecified addresses and the IPv6 multicast and
+// documentation ranges: with no allowlist set, each is refused
+// both when a target is created and when a delivery dials it.
+func TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	guard := delivery.NewTestGuard()
+
+	targets := []string{
+		// The unspecified addresses. On Linux a connection to
+		// either reaches this host's loopback.
+		"http://0.0.0.0:8080/hook",
+		"http://[::]:8080/hook",
+		// IPv6 multicast, all nodes.
+		"http://[ff02::1]/hook",
+		// IPv6 documentation.
+		"http://[2001:db8::1]/hook",
+	}
+
+	for _, target := range targets {
+		t.Run(target, func(t *testing.T) {
+			t.Parallel()
+
+			require.Error(t,
+				guard.ValidateTargetURL(context.Background(), target),
+				"%s must be refused at target creation", target,
+			)
+
+			assertDialRefused(t, guard, target)
+		})
+	}
+}
+
 func TestValidateTargetURL_Allowed(t *testing.T) {
 	t.Parallel()