Bound a run's input by the bus's value limit, 256 KiB (issue 1921)

The lead's ruling on milestone 47/B0: each referenced input is bounded
by the value limit the design already uses.

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/delivery/stepasks_compose_test.go

@@ -95,7 +95,7 @@ func TestAComposeAskIsRefusedWhereItDoesNotBelong(t *testing.T) {
 	}
 	// The asker refuses an input over its bound before it asks, naming its size; the build module refuses bytes that
 	// are not the input named.
-	if _, err := RunInputOf("mesh-delivery-checker.inputs", make([]byte, RunInputBound+1)); err == nil || !strings.Contains(err.Error(), "786433 bytes") {
+	if _, err := RunInputOf("mesh-delivery-checker.inputs", make([]byte, RunInputBound+1)); err == nil || !strings.Contains(err.Error(), "262145 bytes") {
 		t.Errorf("an input over its bound: %v", err)
 	}
 	in := runAsk().Run.Input