fix(mqtt): publish one HA device per node id (refs #872)

[mschabhuettl]

Problem

With multiple ESP32 nodes feeding a single sensing-server, the MQTT
integration publishes one shared Home-Assistant device
(wifi_densepose_wifi-densepose-1) for all of them — contradicting
docs/integrations/home-assistant.md, which states the integration creates one
device per node.

Verified on ghcr.io/ruvnet/wifi-densepose:latest (post-#872, image created
2026-05-31T15:44:26Z): with two distinct nodes powered, only
wifi-densepose-1 ever appears in discovery, and the broker shows a single
client wifi-densepose-1 with entities=20.

Root cause

The per-node data already exists internally — VitalsSnapshot carries
node_id, and the REST /api/v1/nodes endpoint separates nodes correctly.
The MQTT path is what collapses them:

• main.rs builds one OwnedDiscoveryBuilder with a fixed
  node_id = client_id, bridges the aggregate sensing broadcast, and
  stamps that same id onto every snapshot.
• publisher.rs::run holds that single builder for its whole lifetime and
  never reads snap.node_id.

Result: N physical nodes → 1 HA device.

The discovery/topic layer is already fully node-parametric
(DiscoveryBuilder formats every topic as wifi_densepose_{node_id}/...), so
the fix is confined to the publisher loop and the bridge that feeds it.

Fix

• src/mqtt/publisher.rs: maintain a HashMap<node_id, NodeEntry>.
  Register a new HA device lazily (retained discovery + availability) the first
  time a node id is seen on the stream; route each snapshot to its own node's
  builder; per-node rate limiters; re-publish discovery per node on
  refresh/reconnect; publish offline for every known node on shutdown.
• src/main.rs: fan out the sensing broadcast per node — emit one
  VitalsSnapshot per entry of the nodes[] array, preserving real ids,
  instead of stamping a single fixed id. Presence is mapped as
  motion_level != absent && status == active.

Result

N nodes surface as N HA devices (wifi_densepose_1 … wifi_densepose_N),
each with its own presence / person-count / motion entities. New nodes are
auto-registered the first time they appear on the stream. Without a friendly
name they show as "RuView node "; a future --mqtt-zone-map flag can map
ids to room names.

Testing

• A CI workflow (.github/workflows/pr-check.yml) builds the crate with
  --features mqtt on every push to verify compilation. (Added for
  verification; happy to drop it before merge if you prefer.)
• Live: two nodes powered → two distinct discovery devices.

Open question for review

The per-node fan-out in main.rs reads v["nodes"] from the sensing
broadcast, assuming the same shape as REST /api/v1/nodes
(node_id / status / motion_level / person_count / rssi_dbm).
If that broadcast does not carry the per-node array, the bridge should instead
be fed from the node_states map the REST handler already uses. Happy to
adjust based on review.

Refs #872. Fixes #898

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR adds per-node Home Assistant (HA) discovery/publishing by deriving MQTT discovery identity and topics from each inbound VitalsSnapshot.node_id, allowing multiple physical nodes (rooms) to appear as separate HA devices.

Changes:

• Add per-node publisher state (HashMap<node_id, NodeEntry>) with independent availability + rate limiting and lazy discovery/availability registration on first sight.
• Update reconnect/refresh/heartbeat flows to operate across all known nodes.
• Change main MQTT snapshot fan-out to emit one VitalsSnapshot per element in the inbound nodes array, preserving per-node ids.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
v2/crates/wifi-densepose-sensing-server/src/mqtt/publisher.rs	Introduces per-node discovery identities + per-node availability/rate limiting and updates run loop logic accordingly.
v2/crates/wifi-densepose-sensing-server/src/main.rs	Switches MQTT publisher builder to a template and fans out inbound JSON into per-node VitalsSnapshots.

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