feat(server): per-node state pipeline for multi-node sensing (ADR-068, #249)

Replaces the single shared state pipeline with per-node HashMap<u8, NodeState>.
Each ESP32 node now gets independent:
- frame_history (temporal analysis)
- smoothed_person_score / prev_person_count
- smoothed_motion / baseline / debounce state
- vital sign detector + smoothing buffers
- RSSI history

Multi-node aggregation:
- Person count = sum of per-node counts for active nodes (seen <10s)
- SensingUpdate.nodes includes all active nodes
- estimated_persons reflects cross-node aggregate

Single-node deployments behave identically (HashMap has one entry).
Simulated data path unchanged for backward compatibility.

Closes #249
Refs #237, #276, #282

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: ruvnet