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.

docs/CURRENT-STATE.md

@@ -1985,6 +1985,60 @@ the subject (tracking stopped, Satellites off) or tracking another target
 ends the view without refocusing. `src/ui/cockpitOrbitalView.test.mjs`
 covers entry, camera, zoom, look, HUD and exit.
 
+### Application subjects
+
+An application can add its own first-person subjects beside the aircraft,
+vessel and satellite: `services.cockpitSubjects` to `createApplicationControls`,
+a list of subject providers (`src/cockpitSubjectProviders.js`, checked once by
+the coordinator; a malformed one is left out with a warning). A provider names
+its layer, an entry face and labels in the page's language, and supplies four
+operations: `candidate()` (what entry would ride now), `hold(id)`,
+`read(nowMs, ctx)` (a camera frame each update) and `hud(frame, ctx)` (a HUD
+view model), with `release(id, {restore})` on exit. Entry needs the provider's
+layer on, no Context change in flight and the provider's own
+`isEntryAllowed(candidate)`; voice stays aircraft-only and is refused, with
+the reason, while a provider's view is in use.
+
+Entry rides the most recently selected subject of all kinds: a tracked
+aircraft, the tracked satellite, the selected vessel or a provider's
+candidate, by when the Cockpit first saw it; subjects first seen together keep
+the fixed order (aircraft, satellite, vessel, then providers by priority). The
+newest decides alone: when its gate is shut there is no entry rather than an
+older subject's.
+
+A frame places the eye `eye.upM` above `heightM` (the shared floor where the
+frame gives none), slews toward `headingDeg` at the provider's rate, and may
+turn off the floor clamps (`groundClamp: false`, for a subject inside a
+structure) or keep the anchor exactly on the subject (`anchor: 'exact'`, for a
+layer that already moves it smoothly); `cameraIntervalMs: 0` updates the
+camera every rendered frame. A frame that is null, malformed or for another
+subject ends the view (`restoreTracking: false`), as an aircraft tracked
+meanwhile and the provider's layer turning off do; the exit control asks the
+provider to restore its subject. The generic HUD shows the provider's kicker,
+title, meta line, speed with its unit and tag, readout cards, always-visible
+source rows, banner, route card and Live Signals rows, a veil that darkens the
+picture under the HUD (with passing light streaks, which stop under reduced
+motion), arrow-key look-around when the provider asks for it, and a progress
+line along the bottom edge (`src/cockpitProgress.js`: close ticks merge,
+labels never overlap). Core translates none of the provider's text: a host
+words it for the page's language (`@gods-eye-view/core/i18n/locale`:
+`getLocale`, `onLocaleChange`).
+
+The Rail layer offers a read API for such a view: `getSelectedTrain()` and
+`getCockpitSubject()` (a snapshot of the train: where it is along its line,
+which way it runs (`pathBinding.js` learns it), its rail height, speed as
+reported or derived, position source, feed credit and stops),
+`setCockpitSubject(key)` (its marker and card hidden, the selection kept, its
+feed held, clicks and Escape left to the view) and `focusTrain(key)`; its pure
+path module is exported as `@gods-eye-view/core/layers/rail/paths`. Transit
+offers the same for a vehicle (`getSelectedVehicle`, `getCockpitSubject`,
+`setCockpitSubject`, `focusVehicle`): a held vehicle keeps playing when it is
+out of the camera's view, and its sprite, card, trail and detection box are
+hidden. `src/ui/cockpitSubjectView.test.mjs`,
+`src/cockpitSubjectProviders.test.mjs`, `src/cockpitProgress.test.mjs`,
+`src/layers/rail/cockpitSubject.test.mjs` and the Transit lifecycle tests
+cover them.
+
 ## Context coordination
 
 Context controls own Contacts/Space Missions state, entry and exit transactions,