Carry the power draw to the bar over the bus, not through a file (hq ADR 0260)

Modules talk over the bus. The power module's bundle puts each fresh reading of its own
sampler in its state draw, keyed by the machine; the bar's block shows that state, which
the bar's bundle watches under the read the mesh grants it. A new catalogue check refuses
a module naming another module's path under /run or /var/lib.

modules/i3status-rust/cmd/i3status-rust-tools/blocks_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"text/template"
@@ -33,7 +34,7 @@ func blockTemplate(t *testing.T) *template.Template {
 
 func render(t *testing.T, tmpl *template.Template, piece string) string {
 	t.Helper()
-	view := map[string]any{"module": "power"}
+	view := map[string]any{"module": "power", "machine": "laptop"}
 	if err := json.Unmarshal([]byte(piece), &view); err != nil {
 		t.Fatal(err)
 	}
@@ -96,3 +97,24 @@ func TestTheMemoryIsWhatProgramsHoldNotTheCache(t *testing.T) {
 		t.Fatal("the memory block counts the cache as used")
 	}
 }
+
+func TestAContributedStateIsACustomBlockReadingThisMachinesValue(t *testing.T) {
+	tmpl := blockTemplate(t)
+	got := render(t, tmpl, `{"bar":"bottom","place":"status","shows":"state","options":{"state":"power.draw"}}`)
+	want := "[[block]]\nblock = \"custom\"\ncommand = \"~/.local/bin/i3status-value power.draw laptop\"\ninterval = 5\nhide_when_empty = true\n\n"
+	if got != want {
+		t.Fatalf("%q, want %q", got, want)
+	}
+	got = render(t, tmpl, `{"bar":"bottom","place":"status","shows":"state","options":{"state":"power.draw","key":"desk"}}`)
+	if !strings.Contains(got, "i3status-value power.draw desk") {
+		t.Fatalf("%q", got)
+	}
+	// And the bundle finds what it rendered.
+	f := filepath.Join(t.TempDir(), "bottom-bar.toml")
+	if err := os.WriteFile(f, []byte(got), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if w := wantedIn([]string{f}); len(w) != 1 || w[0] != (Wanted{"power.draw", "desk"}) {
+		t.Fatalf("%v", w)
+	}
+}