F3.4 Two-way IGate: messages from the internet to local RF stations #208

Spec: to write first (docs/features/F3.4-….md), from the scope row.

The internet-to-RF half of F3.4: messages (and optionally objects) for stations heard locally are gated to RF with the third-party header, by the rules, rate-limited; the station-heard list on the dashboard.

Gap: G16. Blocked by APRS-NET/APRS.NET#212 (sub-issues #213, #214). Every test uses a local Direwolf with null audio.