fix(rotator): the compass shows the heading it already knows

Opening Station Control left the compass blank for a second or two while the
amplifiers, relay boards and power supply filled at once.

Nothing was slow. The status bar keeps the shared heading loop running, so when
the compass mounts there is already a tick pending — up to the full three-second
idle interval away — and the new subscriber simply waited out the rest of it.
The heading was known the whole time and had nowhere to be read from.

The last heading is now kept and handed to whoever subscribes next. That matters
more on the Alpha SPID this was reported on than it would elsewhere: every poll
is an open, a read at 600 baud and a close, so even fetching immediately on
mount would not have been immediate.

In a microtask, so a subscriber is never called back before subscribeRotorHeading
has returned to it.

changelog.json

@@ -8,7 +8,8 @@
       "Lookup cache: a TTL of 0 switches it off, so a callbook record you are correcting is re-read every time.",
       "TCI radios: when the rig forbids transmitting, PTT says so instead of doing nothing silently.",
       "WAJA carried Japan’s civil prefecture numbers instead of the JARL’s: 35 of the 47 references are renumbered.",
-      "Award references can be renumbered in the editor — the number was the one field it would not let you correct."
+      "Award references can be renumbered in the editor — the number was the one field it would not let you correct.",
+      "The compass fills the moment Station Control opens, instead of waiting out the rest of a polling interval."
     ],
     "fr": [
       "Une entité qui est un seul groupe d’îles remplit désormais la référence IOTA toute seule, sans abonnement callbook.",
@@ -16,7 +17,8 @@
       "Cache des recherches : un TTL à 0 le désactive, pour relire à chaque fois une fiche callbook en cours de correction.",
       "Radios TCI : quand la radio interdit l’émission, le PTT le dit au lieu de ne rien faire en silence.",
       "WAJA portait les numéros civils des préfectures japonaises et non ceux de la JARL : 35 des 47 références sont renumérotées.",
-      "Les références d’un diplôme se renumérotent dans l’éditeur : le numéro était le seul champ qu’il refusait de corriger."
+      "Les références d’un diplôme se renumérotent dans l’éditeur : le numéro était le seul champ qu’il refusait de corriger.",
+      "La boussole se remplit dès l’ouverture de Station Control, au lieu d’attendre la fin d’un intervalle d’interrogation."
     ]
   },
   {