feat: align fast-GPS beacons with MeshUI firmware (16-byte format + cadence parity)

Switch the fast-GPS beacon to the firmware's canonical 16-byte wire format
(magic, key6, lat/lon microdegrees, speed km/h) — no on-wire timestamp; the
receiver stamps its own RX time. Drops compatibility with the older 19-byte app
format.

Mirror the firmware's adaptive send cadence in the app-fallback path
(LocationTrackingService): anchor + dwell motion hysteresis, EMA ground speed,
9-min stationary keepalive, fix-quality (accuracy) gate, and a post-RX hold-off
to avoid channel collisions. Parked nodes now beacon the stable anchor instead
of GPS wander.

Add the fast_gps_region config: region-scope picker in settings backed by the
firmware's fast_gps_region / fast_gps_regions custom vars.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/providers/contacts_provider_test.dart

@@ -762,10 +762,11 @@ void main() {
           senderKey6: '323334353637',
           latitude: 44.123456,
           longitude: 13.654321,
-          timestampSeconds: 1700001234,
+          speedKmh: 12,
         ),
       );
 
+      final nowSeconds = DateTime.now().millisecondsSinceEpoch ~/ 1000;
       final updated = provider.findContactByKey(publicKey)!;
       expect(updated.telemetry, isNotNull);
       expect(updated.telemetry!.gpsLocation, isNotNull);
@@ -780,8 +781,9 @@ void main() {
       expect(updated.telemetry!.batteryMilliVolts, isNotNull);
       expect(updated.advLat, equals((44.123456 * 1e6).round()));
       expect(updated.advLon, equals((13.654321 * 1e6).round()));
-      expect(updated.lastAdvert, equals(1700001234));
-      expect(updated.lastMod, equals(1700001234));
+      // The beacon carries no timestamp; the receiver stamps its own RX time.
+      expect(updated.lastAdvert, closeTo(nowSeconds, 5));
+      expect(updated.lastMod, closeTo(nowSeconds, 5));
       expect(
         provider.estimatedLocationFor(updated.publicKeyHex),
         isNotNull,
@@ -804,7 +806,7 @@ void main() {
           senderKey6: '010203040506',
           latitude: 10,
           longitude: 20,
-          timestampSeconds: 99,
+          speedKmh: 0,
         ),
       );
       final after = provider.findContactByKey(publicKey)!;