fix(mqtt): per-node HA devices use each node's own presence/motion (#872)

[ruvnet]

The bug

#898 made the MQTT bridge emit one Home-Assistant device per node, but it applied the room-level aggregate classification to every node:

let mk = |nid, rssi| VitalsSnapshot { presence /*aggregate*/, motion /*aggregate*/, … };
for node in arr { vtx.send(mk(format!("{node_id}-node{n}"), node["rssi_dbm"])); }

So on a multi-node deployment:

• a node watching an empty corner reported "present" whenever any other node saw a person;
• motion_level: "absent" was mis-mapped to full motion (the aggregate match had no "absent" arm, so it hit Some(_) => 1.0).

The per-node data was already available — each entry in the broadcast nodes array is a PerNodeFeatureInfo carrying its own classification (motion_level/presence/confidence) and rssi_dbm.

The fix

Extracted the JSON→VitalsSnapshot mapping into a pure, testable function vitals_snapshots_from_sensing_json(v, base_id) that:

• reads each node's own classification, deferring to the room aggregate only for fields a node omits;
• keeps vitals (breathing/heart rate) and person count room-level (they aren't per-node);
• maps "absent"/"none"/"still"/"idle" → zero motion;
• falls back to a single aggregate snapshot when there's no nodes array (wifi/simulate).

Tests (new)

4 unit tests under mqtt_bridge_tests:

• per_node_presence_uses_each_nodes_own_classification — node 1 present+moving, node 2 absent → node 2 does not inherit the aggregate
• per_node_missing_fields_fall_back_to_aggregate
• falls_back_to_single_aggregate_when_no_nodes
• absent_motion_level_is_zero_motion

Verification

• cargo build -p wifi-densepose-sensing-server --no-default-features --features mqtt ✓
• new tests pass; full crate unit suite 432 + 114 passed, 0 failed.

Supersedes #899

#899 targeted the same bug but read non-existent fields (node["motion_level"], node["status"] == "active") instead of the nested node["classification"] + stale, so it would have forced every node to "absent". Recommend closing #899 in favor of this.

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