feat(squawk): carry emergencySource + observation time into analyst records

Review on #856 (kvnloo): the analyst projection flattened a 7700 squawk and
the readsb ADS-B emergency field into the same emergency/emergencyKind pair.

- mapAnalystRecord (flights, military) adds emergencySource ('squawk' |
  'ads-b' | null, from aircraftEmergency().source), emergencyBroadcast (new
  pure emergencyBroadcastPhrase: "broadcasting squawk 7700 (general
  emergency)" vs "ADS-B reports general emergency") and observedAtMs (the
  record's lastContactEpochMs: last transponder message, epoch ms).
- emergencySource is a queryable analyst text field; the voice payload passes
  it and emergencyBroadcast through, and observedAtMs only for aircraft with
  a status. Tool description asks the model to narrate with the phrase.
- Source-to-analyst fixture test: OpenSky 7700 vs adsb.lol emergency=general
  on squawk 1200 differ in provenance and timestamp; filterable by source.
- Voice tool-payload pins re-derived for the additive description change.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -103,7 +103,10 @@
   Minimum-fuel and lifeguard statuses are reported but not tinted. The status
   follows the latest poll instead of sticking, so a cleared code stops
   highlighting on the next refresh. The analyst and voice query engine can
-  filter by `squawk`, `emergency` and `emergencyKind`. No new source or
+  filter by `squawk`, `emergency`, `emergencyKind` and `emergencySource`
+  (whether the squawk or the ADS-B field named the status), and each result
+  says when the transponder was last heard, so answers read "broadcasting
+  squawk 7700" or "ADS-B reports general emergency". No new source or
   request: OpenSky already carries the squawk and adsb.lol carries both.
 
 - Street Level: Google Street View is the second provider. On the Google 3D