Add passive presence daemon — multi-signal WiFi/ARP/DHCP/BLE with exponential decay

Implements DevTrack #530: Person Presence Intelligence daemon for BigBrother drop implant.

Core components:
- presence_daemon.py: 4 sensor threads (probe, ARP, DHCP, BLE) with exponential decay
- presence_schema.sql: SQLite schema for persons, devices, signals, occupancy log
- install.sh: Deployment script (copies to /opt/sensor/, creates systemd service)
- AUDIT_impl.md: Implementation notes, limitations, audit checklist

Features:
- Exponential decay model (lambda=0.08/min) for signal certainty
- Presence anchor (45-min hold time prevents false VACANT)
- Person state machine: UNKNOWN → PRESENT ↔ ABSENT
- Multi-signal fusion: max + 0.08×min for device aggregate
- Thread-safe in-memory state with SQLite persistence
- Matrix webhook alerting (if configured)
- HTTP status endpoint on 127.0.0.1:9191
- Graceful BLE degrade if bleak unavailable
- No tool fingerprints in output or service names

presence/presence_schema.sql

@@ -0,0 +1,33 @@
+CREATE TABLE IF NOT EXISTS persons (
+    id INTEGER PRIMARY KEY,
+    name TEXT NOT NULL,
+    notes TEXT
+);
+
+CREATE TABLE IF NOT EXISTS devices (
+    mac TEXT PRIMARY KEY,
+    person_id INTEGER REFERENCES persons(id),
+    label TEXT,
+    added_at REAL DEFAULT (unixepoch())
+);
+
+CREATE TABLE IF NOT EXISTS signals (
+    id INTEGER PRIMARY KEY,
+    mac TEXT NOT NULL,
+    signal_type TEXT NOT NULL,
+    certainty REAL NOT NULL,
+    ts REAL DEFAULT (unixepoch())
+);
+
+CREATE TABLE IF NOT EXISTS occupancy_log (
+    id INTEGER PRIMARY KEY,
+    person_id INTEGER REFERENCES persons(id),
+    old_status TEXT,
+    new_status TEXT NOT NULL,
+    ts REAL DEFAULT (unixepoch())
+);
+
+CREATE INDEX IF NOT EXISTS idx_signals_mac ON signals(mac);
+CREATE INDEX IF NOT EXISTS idx_signals_ts ON signals(ts);
+CREATE INDEX IF NOT EXISTS idx_occupancy_person ON occupancy_log(person_id);
+CREATE INDEX IF NOT EXISTS idx_devices_person ON devices(person_id);