policy/v2: write a single-address via destination bare

Via rules wrote every destination with its prefix bits; ACL and grant
rules already drop them for a single address. Both use one formatter.

hscontrol/policy/v2/compiled.go

@@ -1027,7 +1027,7 @@ func compileViaForNode(
 				destPorts = append(
 					destPorts,
 					tailcfg.NetPortRange{
-						IP:    prefix.String(),
+						IP:    filterIP(prefix),
 						Ports: port,
 					},
 				)

hscontrol/policy/v2/filter.go

@@ -173,16 +173,10 @@ func (pol *Policy) destinationsToNetPortRange(
 
 		for _, pref := range prefixes {
 			for _, port := range ports {
-				pr := tailcfg.NetPortRange{
-					IP:    pref.String(),
+				ret = append(ret, tailcfg.NetPortRange{
+					IP:    filterIP(pref),
 					Ports: port,
-				}
-				// Drop the prefix bits if its a single IP.
-				if pref.IsSingleIP() {
-					pr.IP = pref.Addr().String()
-				}
-
-				ret = append(ret, pr)
+				})
 			}
 		}
 	}
@@ -190,6 +184,16 @@ func (pol *Policy) destinationsToNetPortRange(
 	return ret
 }
 
+// filterIP writes pref as a filter destination: a single address bare,
+// any other prefix with its bits, as Tailscale SaaS writes them.
+func filterIP(pref netip.Prefix) string {
+	if pref.IsSingleIP() {
+		return pref.Addr().String()
+	}
+
+	return pref.String()
+}
+
 // compileFilterRulesForNode compiles filter rules for a specific node.
 func (pol *Policy) compileFilterRulesForNode(
 	users types.Users,

hscontrol/policy/v2/tailscale_grants_compat_test.go

@@ -172,9 +172,6 @@ var grantSkipReasons = map[string]string{
 
 	// Checks keyed <test_id>/<node>/filter: a listed check that matches
 	// fails, so a fix must delete its entry.
-	"via-grant-v63/blastoise/filter": viaDstAddressForm,
-	"via-grant-v63/squirtle/filter":  viaDstAddressForm,
-
 	"grant-dxp-comma-ports/refused": grantPortList,
 
 	// Contiguous pinned addresses; TestViaGrantHACompat compares these
@@ -195,8 +192,6 @@ var grantSkipReasons = map[string]string{
 
 const grantPortList = "GRANT_PORT_LIST: SaaS refuses a comma port list in a grant ip entry (only one port range allowed); headscale accepts it"
 
-const viaDstAddressForm = "FILTER_ADDRESS_FORM: SaaS writes a single-address via destination as a bare address, headscale as a /32 prefix"
-
 // TestGrantsCompat is a data-driven test that loads all GRANT-*.json
 // test files captured from Tailscale SaaS and compares headscale's grants
 // engine output against the real Tailscale behavior.