mounts: take an SMB source's username as a setting and its password as its own secret, and wait for them

The one INI secret smb-credentials could not be given through the desk
prompt, which takes one hidden line, and a missing login was said as a
fault: the first-node gate put the build back and the operator was told
nothing was theirs to do. The username is now the setting smb-users and
each password the one-line secret smb-password-<source>; a source missing
either answers waits through mesh-sdk's health answer (hq ADR 0283,
issue 386, issue 379).

modules/mounts/cmd/mounts/failed_test.go

@@ -43,9 +43,9 @@ func TestAMountThatFailedBecauseItsSourceWasAwayIsCleared(t *testing.T) {
 	f := newSystem(t)
 	f.write(ConfigFile, "shares=library=/srv/library\nsources=games=smb://192.0.2.76/Share@/mnt/games\n"+
 		"server=homeserver\nat=192.0.2.10\nnode=workstation\naccount=operator\n"+
-		"secret=/var/lib/mesh/mounts/smb-credentials.secret\nstate=/var/lib/mesh/mounts\n")
+		"smb-users=games=root\npasswords=/var/lib/mesh/mounts\nstate=/var/lib/mesh/mounts\n")
 	f.write(ExportsFile, exportsAnswer(t, library))
-	f.write("/var/lib/mesh/mounts/smb-credentials.secret", "[games]\nusername=root\npassword="+testPassword+"\n")
+	f.write("/var/lib/mesh/mounts/smb-password-games.secret", testPassword+"\n")
 	a := f.applier(time.Date(2026, 10, 9, 14, 0, 0, 0, time.UTC))
 	a.Look(context.Background()) // writes and arms both pairs; neither source answers
 	f.calls = nil