F2.2 Position and status beacons: fixed, decaying and SmartBeaconing-style #196

Spec: to write first (docs/features/F2.2-….md), from the scope row.

• Fixed-interval and decaying beacons for a fixed station, SmartBeaconing-style corner pegging for a moving one (with F3.6 GPS);
• status beacons and PHG; compressed or uncompressed position;
• a next-beacon countdown and Beacon now; gated by F2.1 and F2.9.
• Beacons go to APRS-IS here; per-port RF paths come with F3.3 in 0.7.0.

Gaps: G01, G11. Blocked by APRS-NET/APRS.NET#195 (sub-issues #196, #197).