home-assistant: its broker and its Sonarr, Radarr and Lidarr come from the mesh

Home Assistant reached mosquitto and the three Servarr apps at 127.0.0.1 and a port typed into its
own storage. It now requires mqtt-topic (asking for every topic: discovery and the devices' topics
are its job), sonarr-api, radarr-api and lidarr-api, and a run-once `provisions` step — declared
last, restarted when a binding or pair credential changes — makes Home Assistant's config entries
say what the mesh bound, through Home Assistant's own config flows and never its .storage:

- MQTT: the broker is asked first whether it takes the delivered login; then the integration's
  reconfigure flow sets broker, port, username and password, every other setting sent back as Home
  Assistant pre-filled it, and Home Assistant's own connection test must pass. A digest of what was
  written makes a rerun "already as the mesh says". Refused anywhere, nothing is written and Home
  Assistant keeps the login it has.
- Sonarr/Radarr/Lidarr: the bound key is tried against the app (a minted key is never written; the
  failure names the `secret accept`); no entry is made through the user flow; a reauth Home
  Assistant started is finished with the bound key (and URL where the integration asks); an entry
  already at the bound URL, or at another URL reaching the same running app, is left as it is.
  These integrations have no reconfigure flow, so a working entry elsewhere is refused loudly —
  the step never removes an entry.

The sidecar's URL now uses the port it was given.

modules/home-assistant/provisions/mesh.ts

@@ -0,0 +1,42 @@
+// What the mesh delivered to home-assistant's provisions step, and the step's own small memory.
+//
+// Per provision it requires, the mesh writes two files beside each other (the manifest's `binds` and
+// `secrets`): `<provision>.json`, the binding — where the provider is (`at`), what it serves (`port`,
+// `scheme`, …) and the login this module presents (`as`) — and `<provision>.secret`, the pair
+// credential. Nothing here guesses a host, a port or a key.
+
+import { mkdir, readFile, rename, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+
+import type { Binding, Marks } from "./connections.js";
+
+/** A file the mesh wrote, or undefined when it is not there. */
+export async function readIfThere(path: string | undefined): Promise<string | undefined> {
+  if (!path) return undefined;
+  return readFile(path, "utf8").catch(() => undefined);
+}
+
+/** A binding file parsed, or undefined when absent or not JSON. */
+export async function readBinding(path: string): Promise<Binding | undefined> {
+  const raw = await readIfThere(path);
+  if (raw === undefined) return undefined;
+  try {
+    return JSON.parse(raw) as Binding;
+  } catch {
+    return undefined;
+  }
+}
+
+export function marksIn(dir: string): Marks {
+  return {
+    async get(name) {
+      return (await readIfThere(join(dir, `${name}.digest`)))?.trim() || undefined;
+    },
+    async set(name, value) {
+      await mkdir(dir, { recursive: true, mode: 0o700 });
+      const path = join(dir, `${name}.digest`);
+      await writeFile(`${path}.tmp`, `${value}\n`, { mode: 0o600 });
+      await rename(`${path}.tmp`, path);
+    },
+  };
+}