status --json, so a board has something to read

A board reads through interfaces and holds nothing. Everything it needs
is already answered — as text, for people, which is not something a page
can read.

`--json` rather than a serving API, because nothing needs one yet:
whatever serves a board runs the command, and the constraint holds either
way — the board never touches a context's store. An API is the larger
thing and should wait until something asks for it.

Both forms are gathered from the same reads before either says anything,
so they answer the same questions rather than being two implementations
that can drift. That was not true of the first version: the JSON printed
after the text, because the branch was too late.

Four properties, each asserted and each confirmed to fail when removed:

- refused and failed stay distinct all the way out. They are fixed in
  different places, so one word for both sends half a page's readers to
  the wrong one — and how much DID apply is carried, since "three of
  eight" and "none of eight" are different machines
- a machine that never spoke carries no time at all, rather than a zero
  one that any page would format as a date in 1970
- nothing is null. A page distinguishing "no machines are wrong" from
  "this field is missing" has to handle both, and null is the one that
  gets forgotten
- no field is named like a secret. Everything here comes from records
  that hold no readable one, but a shape a page is built against is
  exactly where one would eventually be added for convenience

cmd/mesh-builder/where_test.go

@@ -74,3 +74,44 @@ func TestNeitherIsRefusedRatherThanGuessed(t *testing.T) {
 		t.Fatal("a builder with nowhere to publish reported somewhere")
 	}
 }
+
+func TestTheCredentialComesFromAFileTheMeshSealed(t *testing.T) {
+	// A builder that is a module is given its credential the way every module is: sealed to the
+	// machine and written by the host. An environment variable instead would put the one copy
+	// that matters through a terminal and a process listing.
+	path := filepath.Join(t.TempDir(), "broker")
+	if err := os.WriteFile(path, []byte("amqps://a-builder:secret@broker.internal:5671/\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("MESH_BROKER_FILE", path)
+	t.Setenv("MESH_BROKER_AMQP", "amqp://should-not-be-used@nowhere/")
+
+	got, err := brokerFrom()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "amqps://a-builder:secret@broker.internal:5671/" {
+		t.Fatalf("got %q", got)
+	}
+}
+
+func TestAnEmptyCredentialFileIsRefused(t *testing.T) {
+	// Otherwise the builder connects as nobody and is refused, with the reason three layers away.
+	path := filepath.Join(t.TempDir(), "broker")
+	if err := os.WriteFile(path, []byte("\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("MESH_BROKER_FILE", path)
+	t.Setenv("MESH_BROKER_AMQP", "")
+	if _, err := brokerFrom(); err == nil {
+		t.Fatal("an empty credential was accepted")
+	}
+}
+
+func TestABuilderWithNoCredentialAtAllSaysSo(t *testing.T) {
+	t.Setenv("MESH_BROKER_FILE", "")
+	t.Setenv("MESH_BROKER_AMQP", "")
+	if _, err := brokerFrom(); err == nil {
+		t.Fatal("a builder with no broker reported one")
+	}
+}