Leave a busy container running until it says idle, within 15 minutes

A send that replaced the build agent killed the check it ran, and the queue
started it again elsewhere (issue 594). A container may now declare busy; the
node-engine asks it before replacing it, applies everything else, says the
wait, and replaces it on the first idle pass or at the bound, kept across
restarts (novox/hq ADR 0335).

internal/declaration/declaration.go

@@ -1239,6 +1239,11 @@ type Container struct {
 	// timing, judged by the node-engine beside liveness. Absent: judged alive or not, and nothing more.
 	Health *Health `json:"health,omitempty"`
 
+	// Busy is how this container says it holds work a replacement would lose (novox/hq ADR 0335): asked
+	// before it is replaced, and while it says busy it is left running, within the bound. Absent: never
+	// busy, and replaced at once, as before.
+	Busy *Busy `json:"busy,omitempty"`
+
 	// RunAs is who the container runs as, resolved by the controller from the module's identity and the
 	// image's declared interface (novox/hq ADR 0325 §2, §3): told by environment (the image reads PUID and
 	// PGID, say) or by the runtime's own user option. Part of the spec, so a change recreates it. Absent: the
@@ -1318,6 +1323,7 @@ func (c *Container) Target() string   { return c.Name }
 
 func (c *Container) validate(where string, _ bool) []string {
 	problems := c.Health.problems(where, true, !c.RunOnce && c.Schedule == "")
+	problems = append(problems, c.Busy.problems(where, !c.RunOnce && c.Schedule == "")...)
 	if c.Name == "" {
 		problems = append(problems, where+": a container needs a name")
 	}