docs: add presence subsystem reference

docs/architecture/subsystems/presence.md

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+# presence
+
+## Purpose
+
+Tracks whether household members are home by ICMP-pinging their phones on the LAN at a fixed interval. Derives an overall household state (EMPTY, OCCUPIED, etc.) from per-member presence and publishes both to MQTT so the rule engine and arm-state FSM can react.
+
+## Key files
+
+- `vigilar/presence/monitor.py` — `PresenceMonitor`: ping loop, per-member state machine with `departure_delay_m` grace period, publishes per-member and household-level topics
+- `vigilar/presence/models.py` — `MemberPresence` dataclass and `derive_household_state` aggregator
+- `vigilar/presence/__init__.py`
+
+## MQTT topics
+
+**Subscribes:** none
+**Publishes:**
+- `vigilar/presence/{name}` — per-member status (`HOME` or `AWAY`, plus role)
+- `vigilar/presence/status` — aggregated household state and a `{name: HOME|AWAY}` map
+
+## Database tables
+
+none — presence state lives only in memory and on the bus.
+
+## Depends on
+
+- LAN reachability of configured `[presence.members]` IPs
+- `ping` binary on the host
+
+## Consumed by
+
+- `events` — rules and the arm-state FSM react to household state changes
+- `alerts` — smart alert profile matcher in `vigilar/alerts/profiles.py` branches on `HouseholdState`
+
+## Notes
+
+A member is marked HOME the moment a ping succeeds, but only marked AWAY after `departure_delay_m` minutes without a successful ping — this prevents a single dropped reply from flipping the household state. The monitor uses `time.monotonic()` for the grace window, so it is immune to wall-clock jumps. Per-member and household topics are republished every poll, not only on transitions, so late subscribers always get current state.