[gx1400]

Second part of #314 (F1.41 behavior 7). Stacked on #458.

• When: Engine.RunAsync restores first, before any source connects.
• What it reads: the packets kept from the last RemoveAfter (2 h by default; at most 24 h, which also applies when stations are never removed), in order and with their original times.
• What it rebuilds:
  • The station store and tracks, through StationStore.Apply. Same-source duplicates are listed but not applied, as when they arrived.
  • The packet list: the last RecentPacketCapacity packets, with own packets marked Sent.
  • A sweep then marks stations stale or removes them as they'd be had the engine kept running.
• What it doesn't touch: messaging, acks, alerts, digipeating, IGating, transmitting, the dashboard's counts and PacketsReceived. Nothing is written to the history again.
• Spec change: the restart reports itself in the log ("Restored N packets from the last M minutes") instead of a status line. It finishes before anything connects, so a status would never be seen.

Tests: HistoryRestoreTests checks three things:

• stations, tracks and the packet list come back, and a station older than the window doesn't;
• nothing is counted or kept again;
• with history off, nothing is restored.

Release build, all tests and csharpier pass locally.