Say why assign finds no module, and keep an assignment pending on its build

A merge asks for a new module's build, and assign answered "no module of that
name" until the build registered it, which read as a module nobody registered
(hq issue 325). Keep every build request, tell a build in flight, a module
known and not built, and an unknown name apart, and make an assignment made
while the build runs when the build registers the module.

cmd/mesh-controller/readable.go

@@ -101,6 +101,24 @@ type meshStatus struct {
 	// Overflowing is every module whose identity overflows the bound of a provision it requires, and
 	// so is left out of its provider's grants (novox/hq ADR 0225). Absent when every identity fits.
 	Overflowing []catalogue.Overflow `json:"overflowing,omitempty"`
+	// Pending is every assignment waiting for its module's build to register it, and every one ended in
+	// the last day with what ended it (novox/hq issue 325). Absent when there is none; PendingUnread says
+	// why they could not be read.
+	Pending       []pendingStatus `json:"pending,omitempty"`
+	PendingUnread string          `json:"pendingUnread,omitempty"`
+}
+
+// pendingStatus is one pending assignment as status says it.
+type pendingStatus struct {
+	Node       string     `json:"node"`
+	Module     string     `json:"module"`
+	State      string     `json:"state"`
+	Build      string     `json:"build"`
+	Repository string     `json:"repository,omitempty"`
+	Path       string     `json:"path,omitempty"`
+	Since      time.Time  `json:"since"`
+	Settled    *time.Time `json:"settled,omitempty"`
+	Note       string     `json:"note,omitempty"`
 }
 
 // machineFiltered is one rule set on a converged machine that the mesh did not write and that
@@ -240,6 +258,11 @@ func statusAsJSON(asked answers) ([]byte, error) {
 	out.Conditions, out.ConditionsUnread = inBrief(asked.conditions), asked.conditionsUnread
 	out.Failing = providerStandings(asked.conditions)
 	out.Overflowing = asked.overflowing
+	out.PendingUnread = asked.pendingUnread
+	for _, p := range asked.pending {
+		out.Pending = append(out.Pending, pendingStatus{Node: p.Node, Module: p.Module, State: p.State,
+			Build: p.Build, Repository: p.Repository, Path: p.Path, Since: p.Since, Settled: p.Settled, Note: p.Note})
+	}
 	for name := range asked.refused {
 		out.Unresolved = append(out.Unresolved, machineUnresolved{
 			Node: name, Problem: asked.refused[name]})