fix(cockpit): a provider's subject rides where it is drawn, and the documented provider rides a moving train

- A frame with no speedMps gets an exact anchor: an inertial one ran on at
  zero speed and closed at 0.75 m/s, so the camera fell kilometres behind a
  moving train. The docs example now passes speedMps and anchor: 'exact',
  a name for its HUD, and isEntryAllowed for a train bound to its line.
- Transit's snapshot height is the floor where playback draws the vehicle,
  not the newest report's floor uphill, and null while that floor is cold,
  so a view waits for the surface instead of standing on the ellipsoid.
- Rail's direction follows playback once it has placed the train: a report
  that runs back (a reversal, or GPS jitter) no longer turns the view
  while the drawn train still runs forward.
- A frame's id is trimmed as the candidate's is.
- A double-click on the selected vessel no longer enters an application's
  subject that took entry after it.
- The docs import from gods-eye-view/..., the package's name.

src/ui/cockpitInput.js

@@ -41,6 +41,9 @@ function subjectWorldPosition(kind) {
   }
   if (kind === 'satellite')
     return this.services.satellitesLayer?.getCockpitPosition?.() ?? null;
+  // An application's subject has no position before its view is entered:
+  // the selected vessel's sprite must not stand in for it.
+  if (kind !== 'vessel') return null;
   const info = this.readVesselInfo();
   if (!Number.isFinite(info?.latitude) || !Number.isFinite(info?.longitude))
     return null;