feat(layers): highlight aircraft broadcasting a transponder emergency

Flights and Military Flights tint aircraft red while the latest poll
carries squawk 7500/7600/7700 or an adsb.lol ADS-B emergency status
(general/unlawful/nordo/downed). Minimum-fuel and lifeguard priorities
are reported but not tinted. The Cockpit HUD aircraft-meta line names
the broadcast code in red. The status is not sticky: a cleared code
stops highlighting on the next refresh.

The analyst/voice query engine gains squawk, emergency and
emergencyKind fields. No new source or request: OpenSky state vectors
already carry the squawk and adsb.lol/readsb carries both fields.

Built by the Hex project (Crew Leo Agile dev team).

Co-Authored-By: Claude <[email protected]>

src/data/aircraftEmergency.js

@@ -0,0 +1,71 @@
+// src/data/aircraftEmergency.js
+/**
+ * Transponder emergency status for one aircraft, from the Mode A squawk and
+ * the ADS-B emergency/priority field.
+ *
+ * Emergency squawks are ICAO-standard worldwide: 7500 unlawful interference,
+ * 7600 radio failure, 7700 general emergency. readsb (adsb.lol) also reports
+ * `emergency`, "a superset of the 7x00 squawks": none, general, lifeguard,
+ * minfuel, nordo, unlawful, downed, reserved. OpenSky state vectors carry the
+ * squawk only.
+ *
+ * The status is what the transponder broadcast in THIS poll, never a finding:
+ * crews pass through 7x00 codes while dialling, and a real emergency clears
+ * when the code does. Callers keep it per poll (not sticky).
+ */
+
+/** @constant {string} Tint for aircraft broadcasting an emergency (not a priority). */
+export const EMERGENCY_TINT_CSS = '#FF4444';
+
+const SQUAWK_KINDS = { 7500: 'unlawful', 7600: 'nordo', 7700: 'general' };
+
+const KINDS = {
+  general: { label: 'General emergency', severity: 'emergency' },
+  unlawful: { label: 'Unlawful interference', severity: 'emergency' },
+  nordo: { label: 'Radio failure', severity: 'emergency' },
+  downed: { label: 'Downed aircraft', severity: 'emergency' },
+  minfuel: { label: 'Minimum fuel', severity: 'priority' },
+  lifeguard: { label: 'Lifeguard (medical priority)', severity: 'priority' },
+};
+
+/**
+ * A Mode A code as its four octal digits, else null.
+ * @param {unknown} value
+ * @returns {string|null}
+ */
+export function normalizeSquawk(value) {
+  const code = String(value ?? '').trim();
+  return /^[0-7]{4}$/.test(code) ? code : null;
+}
+
+/**
+ * Emergency status from one observation. An explicit ADS-B status wins; a
+ * 7x00 squawk counts even when the ADS-B field says `none`.
+ * @param {{squawk?: unknown, emergency?: unknown}} [observation]
+ * @returns {{kind: string, label: string, severity: 'emergency'|'priority',
+ *   squawk: string|null, source: 'ads-b'|'squawk'}|null}
+ */
+export function aircraftEmergency({ squawk, emergency } = {}) {
+  const code = normalizeSquawk(squawk);
+  const reported = String(emergency ?? '')
+    .trim()
+    .toLowerCase();
+  const fromAdsb = Object.hasOwn(KINDS, reported);
+  const kind = fromAdsb ? reported : (SQUAWK_KINDS[code] ?? null);
+  if (!kind) return null;
+  return {
+    kind,
+    ...KINDS[kind],
+    squawk: code,
+    source: fromAdsb ? 'ads-b' : 'squawk',
+  };
+}
+
+/**
+ * True when the aircraft is broadcasting an emergency (not a priority status).
+ * @param {{severity?: string}|null|undefined} status
+ * @returns {boolean}
+ */
+export function isEmergency(status) {
+  return status?.severity === 'emergency';
+}