[gx1400]

Closes #386. The bottom of tonight's stack.

The Dashboard's counters (F1.22) and the GPS statistics (F3.14) were memory-only, so a restart, an update or a crash emptied them. As you chose, they're now kept in a file; SQLite can take its place later.

• dashboard-history.json in the data folder (on the gateway, its data volume). It's a versioned JSON HistorySnapshot behind an internal IHistoryStore, so a SQLite store can replace FileHistoryStore without touching the dashboard.
• When it's saved: every 5 minutes (EngineOptions.HistorySaveInterval), as the engine stops, and right after Clear packets, so a restart doesn't bring the cleared counts back. Each save writes .tmp and renames it over the old file.
• At start: whatever is under 24 hours old is loaded before anything is counted.
  • The time the app was down shows as empty points.
  • "Counting since" keeps the first run's start.
  • An outage still going on when the app stopped ends at the save.
  • "Now" values wait for a new sample.
• What's kept: DashboardCounters and GpsHistory gain Export/Import. Enum values are stored by name, so reordering an enum can't misread an old file. That covers the minute buckets, the distinct-station rings, per-station tallies (so My view still works), relays, sources, the newest stations, and the GPS minutes, outages and wander origin. A 24-hour file is a few hundred KB at most.
• A damaged or other-version file is renamed .invalid, and counting starts afresh with a log warning.
• Tests stay out of real folders. EngineOptions.HistoryFile is set only when the engine is added to a host, so tests that build the engine themselves keep history in memory. HostingTests.EnabledEngineStartsConnectingWithTheHost ran the hosted engine on the real data folder and wrote a history file there during my first run. It now uses a temporary folder; I deleted the stray file and checked that a full run leaves nothing.
• Specs: F1.22 decision 3 and F3.14 behavior 6 now say history is kept.

Tests (HistoryFileTests, 5):

• dashboard counts survive a JSON round trip and a restart, including My view;
• what's over 24 hours old is dropped;
• GPS history comes back, with an ongoing outage ended and down time empty;
• a damaged file and a wrong version are set aside, and the save leaves no .tmp;
• an engine saves on stop and a new engine takes the history up.