docs(changelog): add the #1686 fragment

changelog.d/1686-cockpit-subjects.md

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+- Let an application add its own Cockpit subjects through
+  `services.cockpitSubjects`. A provider supplies the subject's position,
+  heading and HUD (in the page's language, via the new `./i18n/locale`
+  export); the camera can follow it without the ground clamp, anchored
+  exactly on it, and a generic HUD adds source rows, a banner, a veil (off
+  under reduced motion) and a bottom progress line. The Rail and Transit
+  layers expose read and hold APIs so a selected train or vehicle can be
+  ridden, and `./layers/rail/paths` is exported; `docs/APPLICATION.md` shows
+  a minimal provider. GEV itself registers none. Cockpit entry now goes to
+  the most recently selected subject, so a vessel selected after an aircraft
+  is tracked enters BRIDGE (Denis Groshev, @ZloyS, #1686).