feat(sensing-server): EventBus + MQTT bridge for Home Assistant

[schwarztim]

Motivation

RuView deployments need to publish sensing events (presence, body tracking, zones, vitals) to home-automation systems. MQTT is the de-facto protocol for Home Assistant, Node-RED, openHAB, and others. This PR adds two coupled pieces:

1. EventBus (event_stream.rs) — an in-process pub-sub built on tokio::sync::broadcast with a typed Event enum covering presence, body, zone, and vital-sign events.
2. MQTT bridge (mqtt_bridge.rs) — opt-in tokio task subscribing to the EventBus and publishing to MQTT topics under a configurable prefix.

Why bundled into one PR?

The MQTT bridge depends on the EventBus to source events. EventBus was net-new in this fork and is not yet present in upstream's v2/. Splitting into two PRs would require shipping the EventBus first as a no-op (no consumer), which is harder to review and provides no immediate value. Keeping them together produces a complete, testable feature.

Changes

• NEW: v2/crates/wifi-densepose-sensing-server/src/event_stream.rs (+288 LOC) — EventBus, Event enum, EventSubscriber helper
• NEW: v2/crates/wifi-densepose-sensing-server/src/mqtt_bridge.rs (+212 LOC) — MqttBridge task using rumqttc
• v2/crates/wifi-densepose-sensing-server/src/main.rs (+19 LOC) — mod declarations + instantiate EventBus at startup; spawn MqttBridge if MQTT_URL is set; add mqtt_url field to Args
• v2/crates/wifi-densepose-sensing-server/src/cli.rs (+4 LOC) — mqtt_url field in public Args struct
• v2/Cargo.toml + crate Cargo.toml — add rumqttc workspace dependency

Configuration

• SENSING_MQTT_URL / --mqtt-url (required to enable, e.g. mqtt://broker.local:1883)
• MQTT_TOPIC_PREFIX (default ruview/)
• MQTT_USERNAME / MQTT_PASSWORD (optional)

Test fixture URL is mqtt://test.example.com:1883 — no real IPs.

Test results

• cargo test --workspace --no-default-features: 1639 passed, 0 failed (4 new unit tests in mqtt_bridge)
• python v1/data/proof/verify.py: VERDICT: FAIL — pre-existing, also fails on unmodified origin/main (numpy/scipy version drift); not caused by this PR

Notes

• Opt-in: bridge dormant unless --mqtt-url / SENSING_MQTT_URL is set. Zero impact on existing deployments.
• EventBus is also useful as a foundation for future webhook/SSE/REST stream APIs.
• No new ADR — follows existing module patterns. Could be referenced as the "event-bus + integration bridge" pattern in a future design doc.