feat(cockpit): enter Cockpit with only the ridden aircraft's layer on

Cockpit entry no longer needs Contacts or both flight feeds: a civilian
aircraft needs Live Flights, a military one Military Flights. Contacts and
every other layer can be turned on or off before or during a session, and
turning off the ridden layer ends it. A Context mode change in flight still
blocks entry.

When the other air feed is off, Live Signals and the Contact card say
FLIGHTS FEED OFF or MILITARY FEED OFF · PARTIAL PICTURE instead of
presenting a partial picture as complete. Voice refuses entry with
subject-layer-off rather than contacts-inactive, and its tool description
says Contacts is not required.

Double-clicking the followed aircraft enters Cockpit through the same gate.
The Cockpit takes the scene's double-click before Cesium's stock handler,
which would otherwise untrack a followed aircraft whose model is a
primitive. The entry also shows under the Intel HUD readout, sharing the
Contacts button's face. A test keeps the classes the Cockpit adds to the
body from matching another element's rule.

src/contextModePolicy.js

@@ -411,30 +411,37 @@ export function contextRestoreLayerIds(snapshot = {}) {
   return restored;
 }
 
+/** The layers an aircraft Cockpit subject can come from. */
+export const COCKPIT_AIRCRAFT_LAYER_IDS = Object.freeze([
+  'flights',
+  'military',
+]);
+
 /**
- * Cockpit entry belongs to the operational Contacts context bundle. A tracked
- * aircraft alone is not sufficient: both observed-flight feeds must still be
- * active so the cockpit's surrounding-contact picture is not presented as a
- * complete context view when one source is disabled.
+ * Cockpit entry needs only the aircraft it rides: that aircraft's own layer
+ * (Live Flights for a civilian, Military Flights for a military aircraft) must
+ * be on. Contacts and the other feeds are not required. When a source the
+ * surrounding picture uses is off, the Cockpit's Contact card and Live
+ * Signals say the picture is partial instead of blocking entry, so a partial
+ * picture is never presented as complete. A Context mode change in flight
+ * still owns the camera, so entry waits it out.
  *
- * @param {object} input Current context and dependency visibility.
- * @param {string|null} input.contextMode Active Context mode.
- * @param {boolean} input.contextModeChanging Whether the Context transaction is unsettled.
- * @param {boolean} input.flightsEnabled Whether Live Flights is enabled.
- * @param {boolean} input.militaryEnabled Whether Military Flights is enabled.
+ * @param {object} input Current subject and Context state.
+ * @param {string|null} input.subjectLayerId Layer of the tracked aircraft
+ *   ('flights' or 'military'), or null with none tracked.
+ * @param {boolean} input.subjectLayerEnabled Whether that layer is enabled.
+ * @param {boolean} input.contextModeChanging Whether a Context transaction is unsettled.
  * @returns {boolean} Whether cockpit entry may be offered or activated.
  */
 export function cockpitEntryAllowed({
-  contextMode,
+  subjectLayerId,
+  subjectLayerEnabled,
   contextModeChanging,
-  flightsEnabled,
-  militaryEnabled,
 }) {
   return (
-    contextMode === 'flights' &&
-    !contextModeChanging &&
-    Boolean(flightsEnabled) &&
-    Boolean(militaryEnabled)
+    COCKPIT_AIRCRAFT_LAYER_IDS.includes(subjectLayerId) &&
+    Boolean(subjectLayerEnabled) &&
+    !contextModeChanging
   );
 }