Supervises the Improv provisioning processes as a :one_for_all group.

The children are mutually dependent — the Improv manager registers and drives the GattServer + Advert D-Bus exporters (off-loop via the Task.Supervisor) — so a crash of any one must restart all of them together. Under the parent Bluez supervisor's :rest_for_one, the manager (started last) crashing would otherwise leave GattServer/Advert running: the cleartext GATT app + advertisement would stay exported with no session timers or disarm logic.

Restarting the whole group instead drops the exporters' own rebus connections, which makes bluetoothd auto-unregister the GATT application + advertisement, and the manager re-evaluates connectivity from a clean slate.

Mounting contract

Improv is a consumer of the Bluez subtree, not part of it: mount this supervisor in Bluez's extra_children: slot, appended last. Under the Bluez parent's :rest_for_one strategy that position means a bluetoothd/Bluez.Client restart rebuilds the whole Improv group (whose exporters hold now-stale D-Bus registrations), while an Improv fault never disturbs anything before it — the host's scanning/GATT or audio stacks. The child spec's opts are also where the host passes the manager's options (pubsub:, network_type:, ifname:, identify_fun:, device_info:, …) and the advert branding (name_prefix: / local_name:):

{Bluez,
 client: [...],
 extra_children: [
   {Improv.Supervisor,
    [
      pubsub: MyApp.PubSub,
      network_type: &MyApp.Network.type/0,
      name_prefix: "My Device",
      identify_fun: &MyApp.Identify.blink/0,
      device_info: [
        firmware_name: "My Firmware",
        firmware_version: "1.2.3",
        hardware: "Raspberry Pi 3 Model B Plus",
        device_name: "My Device 507f"
      ]
    ]}
 ]}