compat: fix the too-new message's grammar, add a live gate test

The generated refusal read "point at a app within range". Also adds
LiveCompatTest, which checks the half a unit test cannot reach: that two
independently-built artifacts agree on the window, that the profile stays
readable for a version the server refuses, and that both bounds are enforced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/internal/compat/compat.go

@@ -183,7 +183,7 @@ func Check(peer string, r Range, peerName string) (Verdict, string) {
 		return TooOld, fmt.Sprintf("%s %s is older than this build supports (needs %s). Update the %s.",
 			peerName, v, r, peerName)
 	case r.HasMax && !v.Less(r.Max):
-		return TooNew, fmt.Sprintf("%s %s is newer than this build supports (accepts %s). Update this side, or point at a %s within range.",
+		return TooNew, fmt.Sprintf("%s %s is newer than this build supports (accepts %s). Update this side, or use a version of the %s within that range.",
 			peerName, v, r, peerName)
 	}
 	return OK, ""