feat: add no_std arksync-bus foundation

[theredfish]

Summary

This PR introduces arksync-bus as a generic EventBus layer instead of a central crate owning bounded-context payloads.

Current direction:

• arksync-bus owns the generic EventBus infrastructure: envelopes, producer, rules, filters, sinks, and codec helpers.
• Bounded contexts own their published events close to their own infrastructure/events layer, for example arksync-sensor::infrastructure::events::SensorEvent.
• The MVP starts with local in-process delivery so Knot and Hub can communicate without MQTT or a broker.
• MQTT remains a future delivery adapter: the EventBus can later route matching events to a MqttSink backed by minimq and Mosquitto.

EventBus Shape

The rule model separates filtering, delivery, and persistence:

for event in events {
    for rule in rules {
        if !rule.filter(event) {
            continue;
        }

        match rule.persistence() {
            Persistence::Durable => outbox.push(rule, event),
            Persistence::Volatile => {}
        }

        match rule.delivery() {
            Delivery::Local => local_channel.publish(event),
            Delivery::Mqtt { topic } => mqtt.publish(topic, event),
            Delivery::Both => {
                local_channel.publish(event);
                mqtt.publish(topic, event);
            }
        }
    }
}

For this PR, only local volatile delivery is implemented. Persistence and MQTT are intentionally kept as later adapters.

Boundaries

• arksync-bus must not define sensor-specific payloads such as readings, sensor addresses, firmware metadata, or driver details.
• arksync-sensor remains responsible for sensor concepts and published sensor events.
• arksync-knot publishes bounded-context events.
• arksync-hub subscribes locally for the MVP and updates projections/use cases.

Expected MVP flow:

arksync-knot --EventEnvelope<SensorEvent>--> local arksync-bus/channel --> arksync-hub

Validation

• arksync-bus core remains no_std.
• Postcard remains the canonical binary codec for serializable envelopes/events.
• Integration tests cover a local sensor event passing through the EventBus.