feat: family presence ring (who is home, ambient)

[Ryan4n6]

Idea

The four corners of the matrix dim/glow per family member based on whether their phone is on the home network. Glance at the wall, you know who's home — no app, no notification, no GPS. Wordless ambient awareness.

How it works

• Source of truth is the home router: when a known device's MAC has an active DHCP lease (or recent ARP entry), that person is "present."
• Clockwise polls the router (UniFi controller API, or whatever's running) every ~60s for the watched MAC list.
• Per-person config: name, MAC list (a person may have phone + watch + iPad), corner assignment (TL/TR/BL/BR), color.
• Render: 4×4-pixel rounded square in each assigned corner, full brightness when present, dim+desaturated when away. Optional 1-pixel name initial inside.

Privacy story

• No GPS, no location services, no third-party tracking.
• All polling is LAN-local; no data leaves the house.
• Watched MACs configured by the device owner.

Architecture sketch

• New firmware/lib/cw-presence/ module: pluggable backend (UniFi REST, generic ARP-via-router-SSH, OPNsense, etc.) returning {mac → bool present}.
• Settings UI: per-corner person editor (name, color, list of MACs).
• Composable with any clockface: presence ring renders as an overlay on the existing face, not a face of its own. (Implementation note: most current clockfaces don't expose an overlay slot — may need to add IClockface::renderOverlay() as an optional virtual.)

Constraints

• Requires router cooperation. UniFi has a clean API; consumer routers vary. Initial scope: UniFi only, document the backend interface so others can add OPNsense/pfSense/etc.
• MAC randomization: iOS/Android randomize per-SSID by default. The watched MACs are the home-SSID-specific ones, which are stable. Document this so users know which MAC to grab.

Acceptance

• UniFi backend reads device-presence list given controller URL + API key.
• Up to 4 people configurable from the web UI.
• Corners glow/dim within ~60s of presence change.
• No noticeable impact on clock-face animation framerate.
• Failed router poll → corners freeze last-known state, don't all go dark.

Out of scope

• Geofencing / leaving-the-neighborhood alerts.
• Per-person history / who-arrived-when log.
• Multi-network presence (work network, etc.).