Send the bus's user list alone before a walk's gate, so a new grant is on the bus when the first machine is judged (issue 490)

A walk left the machine holding the bus out of a gated send whenever a
build waited there for that same gate, so a module's new health tool was
refused by the bus on its first machine and the gate could not pass. The
grants step sends that machine its declaration with every build kept,
before the first machine's send; plans and the change plan say it.

internal/inventory/plans.go

@@ -305,6 +305,18 @@ type PlanGate struct {
 	// Readings are the judging's readings with their times (novox/hq ADR 0282 decision 6): every reading that
 	// counted a pass, and the first that did not after one that did. At most maxReadings, the newest kept.
 	Readings []GateReading `json:"readings,omitempty"`
+	// Grants is the grants step taken before this gate's send (novox/hq issue 490): the bus's user list sent
+	// alone to the machine holding the bus, every build there kept, so what the send newly grants is on the
+	// bus before the gate judges it. Nil when the list did not change, or the gate's machine holds the bus.
+	Grants *GrantsStep `json:"grants,omitempty"`
+}
+
+// GrantsStep is the bus's user list sent to the machine holding the bus ahead of a gated send (novox/hq
+// issue 490): to which machine, when, and, when it could not be sent, why — the send went on without it.
+type GrantsStep struct {
+	Node   string     `json:"node"`
+	At     *time.Time `json:"at,omitempty"`
+	Failed string     `json:"failed,omitempty"`
 }
 
 // GateReading is one reading of a first-node gate.