mmrs: monitoring streams (state, signal)

[cachebag]

implement real-time d-bus signal subscriptions for modem state and signal quality.

modem state changes

subscribe to Modem.StateChanged(old: i32, new: i32, reason: u32) signal.

pub struct ModemStateChange {
    pub modem_path: String,
    pub old_state: ModemState,
    pub new_state: ModemState,
    pub reason: StateChangeReason,
}

pub async fn monitor_state(&self) -> Result<impl Stream<Item = ModemStateChange> + Send>;

follow the pattern in nmrs/src/monitoring/device.rs.

signal quality

poll SignalQuality property changes via PropertiesChanged signal, or use Modem.Signal interface if extended signal info is needed.

pub struct SignalUpdate {
    pub modem_path: String,
    pub quality: u32,
    pub recent: bool,
    pub access_technology: AccessTechnology,
}

pub async fn monitor_signal(&self) -> Result<impl Stream<Item = SignalUpdate> + Send>;

modem added/removed

subscribe to ObjectManager.InterfacesAdded / InterfacesRemoved on /org/freedesktop/ModemManager1 for hotplug detection.

pub enum ModemEvent {
    Added(String),   // modem path
    Removed(String),
}

pub async fn monitor_modems(&self) -> Result<impl Stream<Item = ModemEvent> + Send>;

stream trait bounds

ensure streams are Send so they work with tokio::spawn. same fix as nmrs #359.