Judge a send on no machine as the core, and test a send carrying the core with the real classes

The review found that a send whose first machine runs none of its
modules would pass at once with no reading, and that every mixed send
in the tests judged its leaf as the core.

cmd/mesh-controller/gate.go

@@ -644,7 +644,14 @@ func judgeMoves(ctx context.Context, open *stores, g *inventory.PlanGate, pairs
 		p, s := gateWants(m, shelf[m])
 		wants, settles = max(wants, p), settles || s
 	}
+	if len(modules) == 0 {
+		// A send judged on no machine of its modules (its first machine not running them) has no class to
+		// be judged by: it is judged as the core, never passed at once without a reading.
+		wants, settles = gatePasses, true
+	}
 	g.Wants = wants
+	// A module's own pass is its own class's: a leaf beside a core module that fails keeps its build after
+	// its one reading, and only what did not pass is put back (issue 318's rule, by class since issue 524).
 	passedAlone := func(m string) bool {
 		p, s := gateWants(m, shelf[m])
 		return (settled || !s) && reading[m] <= healthPerson && g.Healthy[m] >= p