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.

src/layers/rail/rendering.js

@@ -507,6 +507,15 @@ export function createRendering({ state, services, parts }) {
     );
   }
 
+  /**
+   * Whether the train's marker shows: its category is on, and no
+   * first-person view rides it (cockpitSubject.js). A held train keeps its
+   * playback; only its marker hides.
+   */
+  function markerShown(entry) {
+    return categoryShown(entry) && entry.key !== state._cockpitSubjectKey;
+  }
+
   /**
    * A train bound to its railway line is drawn on the line, at the arc its
    * playback has reached; any other train (or a bound train whose reports do
@@ -536,7 +545,7 @@ export function createRendering({ state, services, parts }) {
       undefined,
       scratchPosition,
     );
-    entry.marker.show = categoryShown(entry);
+    entry.marker.show = markerShown(entry);
   }
 
   /** While a train is selected, every other train is drawn see-through. */
@@ -554,7 +563,7 @@ export function createRendering({ state, services, parts }) {
     entry.marker.width = size;
     entry.marker.height = size;
     entry.marker.color = tintFor(entry);
-    entry.marker.show = categoryShown(entry);
+    entry.marker.show = markerShown(entry);
   }
 
   /** Re-tint every train after the selection appeared, moved or cleared. */
@@ -658,7 +667,7 @@ export function createRendering({ state, services, parts }) {
   function cullSelected() {
     const entry = state._trains.get(state._selectedKey);
     if (!entry?.marker) return;
-    const shown = categoryShown(entry) && facesCamera(entry.marker.position);
+    const shown = markerShown(entry) && facesCamera(entry.marker.position);
     if (entry.marker.show !== shown) entry.marker.show = shown;
     parts.routes.cullStops(facesCamera);
   }
@@ -697,7 +706,7 @@ export function createRendering({ state, services, parts }) {
   function applyCategoryVisibility() {
     for (const entry of state._trains.values()) {
       if (!entry.marker) continue;
-      entry.marker.show = categoryShown(entry);
+      entry.marker.show = markerShown(entry);
       schedulePlayback(entry);
     }
     syncRenderHold();