feat(cockpit): an application can add its own cockpit subjects; the Rail and Transit layers expose the read and hold APIs such a subject needs

GEV itself registers no new subject: with no provider, the Cockpit offers
aircraft, satellites and vessels as before, except that the most recently
selected of them now wins entry.

- services.cockpitSubjects: providers ({kind, layerId, entryFace, priority,
  isEntryAllowed, read, hold, release, hud}) are checked once by the
  coordinator; frames and HUD views on every update; a throwing provider
  ends its view or keeps the last HUD.
- Entry: the most recently selected subject wins, across all kinds.
- Camera: a frame may turn the ground clamp off, anchor exactly on the
  subject and update every frame.
- A generic HUD: kicker, title, cards, always-visible source rows, banner,
  a veil with light streaks (off under reduced motion) and a bottom
  progress line whose close ticks merge.
- Text comes from the provider, already in the page's language
  (./i18n/locale: getLocale, onLocaleChange).
- Rail and Transit read and hold APIs (getSelectedTrain/Vehicle,
  getCockpitSubject, setCockpitSubject, focusTrain/Vehicle); a held
  train's marker, card and route line are hidden, and its feed keeps
  polling while held.
- ./layers/rail/paths is exported.

style.css

@@ -9,6 +9,7 @@
 @import './src/ui/styles/status.css';
 @import './src/ui/styles/cockpit.css';
 @import './src/ui/styles/cockpitBridge.css';
+@import './src/ui/styles/cockpitSubject.css';
 @import './src/ui/styles/cockpitRadar.css';
 @import './src/ui/styles/cockpitOrbital.css';
 @import './src/ui/styles/cockpitOrbitalFocus.css';