A command opens each store once, not once per machine

Working out what a machine should be reaches the identity context for its
certificate and the licence context for its model access. Both were opened —
and waited on — inside functions called for every node in a push. Two machines
hid it. Fifty would be fifty connect-and-wait cycles for data that does not
change while the push runs.

So a command holds what it has open, and passes it. Each context is opened on
first use rather than up front, because most commands need one and paying to
reach three would be the same waste from the other side.

The contexts stay separate, which is the point: this is one struct holding
three connections to three databases, not one connection to a shared one. No
context reaches another's store, and each still holds only its own credential
(novox/hq ADR 0008).

A pure move again — the gate is green before and after, and no test changed.

cmd/mesh-control/build.go

@@ -98,11 +98,12 @@ func buildsCommand(ctx context.Context, args []string) error {
 		return errors.New("builds [<module>] [-n N]")
 	}
 
-	inv, err := openInventory(ctx)
+	open, err := openStores(ctx)
 	if err != nil {
 		return err
 	}
-	defer inv.Close()
+	defer open.Close()
+	inv := open.inventory
 
 	builds, err := inv.Builds(ctx, module, *limit)
 	if err != nil {
@@ -262,11 +263,12 @@ func builderCommand(ctx context.Context, args []string) error {
 // reason one broken module no longer blocks a machine's whole declaration: a mesh where one bad
 // repository holds back nine good ones is a mesh where nobody dares add the tenth.
 func buildBehind(ctx context.Context, wait time.Duration) error {
-	inv, err := openInventory(ctx)
+	open, err := openStores(ctx)
 	if err != nil {
 		return err
 	}
-	defer inv.Close()
+	defer open.Close()
+	inv := open.inventory
 
 	held, err := inv.Catalogued(ctx)
 	if err != nil {
@@ -350,11 +352,12 @@ func buildOne(ctx context.Context, repository, ref string, wait time.Duration) e
 	// Kept before it is judged. A failed build that leaves no trace is indistinguishable from one
 	// nobody asked for, and the difference is the whole of whether somebody should be looking at
 	// something.
-	inv, err := openInventory(ctx)
+	open, err := openStores(ctx)
 	if err != nil {
 		return err
 	}
-	defer inv.Close()
+	defer open.Close()
+	inv := open.inventory
 	if err := inv.RecordBuild(ctx, buildFrom(result)); err != nil {
 		return err
 	}