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.

package.json

@@ -167,6 +167,7 @@
     "./ui/cockpit/math": "./src/cockpitMath.js",
     "./ui/cockpit/layout": "./src/cockpitUtilityLayout.js",
     "./ui/cockpit/vision": "./src/cockpitVisionPolicy.js",
+    "./i18n/locale": "./src/i18n/hostLocale.js",
     "./ui/scenes": "./src/ui/scenes.js",
     "./ui/shell": "./src/ui/applicationShell.js",
     "./ui/styles": "./src/ui/styles.js",
@@ -290,6 +291,7 @@
     },
     "./sources/rail-service": "./src/sources/railService.js",
     "./layers/rail/source": "./src/layers/rail/source.js",
+    "./layers/rail/paths": "./src/data/railPaths.js",
     "./sources/map-features": "./src/sources/overpassFeatures.js",
     "./server/providers/mapillary": {
       "node": "./server/providers/mapillary.js"