A build seat's puller takes nothing while its module finds its node cannot build, and says why

A node whose module proves a need of the machine missing (ace's /dev/net/tun, 2026-10-28) pulled
image asks and failed each one; judged ineligible, its capacity is 0 with the reason, as build hours
are, so the asks go to a node that can build them (issue 1126 note).

go/buildstep/pull.go

@@ -86,6 +86,13 @@ type Puller struct {
 	// mesh-config resolves for `build-hours` takes its place once handed. Outside the window the capacity judged by is 0:
 	// steps running finish, no new one is taken; the puller wakes at the window's next edge. Nil builds at any hour.
 	BuildHours func() (*BuildWindow, string)
+	// Eligible says why this node cannot run the seat's asks at all now, and "" when it can: what the module proves of
+	// its machine and finds missing (mesh-build-image: /dev/net/tun cannot be opened, so buildah's network for a RUN line
+	// cannot be made; 2026-10-28 on ace). Read at every judgement, so it is cheap (a module answers its last look).
+	// While it says a reason the capacity judged by is 0, said with the reason: steps running finish, no new one is
+	// taken, and the asks go to a node that can run them instead of failing here. The module calls Wake when its answer
+	// changes. Nil is always eligible.
+	Eligible func() string
 	// Has says whether this node has a need an ask names (delivery.AskNeeds, issue 890). An ask needing what it lacks is
 	// declined: its step ends refused as declined (delivery.StepDeclined), the message is dropped for its asker to put
 	// again, and the worker it came from is not looked at again until the next pull's wait, so a node lacking the need
@@ -291,6 +298,13 @@ func (p *Puller) CapacityDetail() delivery.Capacity {
 			c.Why = fmt.Sprintf("%s; within its build hours %s until %s", c.Why, c.BuildHours, w.NextEdge(at).Format("15:04"))
 		}
 	}
+	if p.Eligible != nil {
+		if why := p.Eligible(); why != "" {
+			c.Ineligible = why
+			c.Why = fmt.Sprintf("0, not eligible: %s (otherwise %s)", why, c.Why)
+			c.Effective = 0
+		}
+	}
 	p.sayCapacity(c)
 	return c
 }
@@ -326,7 +340,8 @@ func (p *Puller) sayCapacity(c delivery.Capacity) {
 	p.mu.Lock()
 	same := p.hasJudged && p.judged.Effective == c.Effective && p.judged.Configured == c.Configured && p.judged.ByCPU == c.ByCPU &&
 		p.judged.CoresPerStep == c.CoresPerStep && p.judged.QuotaCores == c.QuotaCores && p.judged.OutsideHours == c.OutsideHours &&
-		p.judged.BuildHours == c.BuildHours && p.judged.BuildHoursFault == c.BuildHoursFault
+		p.judged.BuildHours == c.BuildHours && p.judged.BuildHoursFault == c.BuildHoursFault &&
+		p.judged.Ineligible == c.Ineligible
 	if !same {
 		p.judged, p.hasJudged, p.capWritten = c, true, false
 	}