feat(flights): a tracked flight's trail splits at gaps, follows the drawn aircraft, and a lost aircraft coasts then reads SIGNAL LOST

The trail no longer joins a day of history into one line. It lifts the pen
across a gap of more than 48 minutes, an implied speed over 1,500 kt, or a
report without altitude (a parked aircraft within 0.1 nm for under 3 h stays
joined), and every history point is kept as its own segment. History never
gets an invented altitude. The live end is the path the aircraft was drawn
along, thinned to its shape, so it no longer cuts corners through turns; a
late report trims what was drawn after it. Old segments are frozen in chunks,
so a 2,000-point trail costs about 0.1 ms a frame.

A tracked aircraft that misses an update shows STALE and is dimmed. It keeps
flying on its last speed and heading for five minutes (at most 1,500 kt),
then stops, dimmed, labelled SIGNAL LOST with the time since its last report,
and is kept for ten minutes after that report so the camera keeps its target.
Aircraft on or near known ground hold at their last report instead of
coasting. Military flights follow the same rules.

scripts/package-boundaries.json

@@ -1459,6 +1459,7 @@
       "src/layers/flights/state.js",
       "src/layers/flights/testing.js",
       "src/layers/flights/tracking.js",
+      "src/data/trailContinuity.js",
       "src/layers/flights/records.js",
       "src/layers/flights/recordPolicy.js",
       "src/layers/flights/snapshotRenderer.js",
@@ -1523,6 +1524,7 @@
       "src/layers/military/state.js",
       "src/layers/military/testing.js",
       "src/layers/military/tracking.js",
+      "src/data/trailContinuity.js",
       "src/layers/military/records.js",
       "src/layers/military/recordPolicy.js",
       "src/layers/military/snapshotRenderer.js",
@@ -1542,6 +1544,7 @@
       "src/i18n/locales/uk.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
+      "src/ui/unitLabels.js",
       "src/time/worldClock.js",
       "src/time/julian.js"
     ],
@@ -2632,6 +2635,7 @@
       "src/data/trafficPresetStyle.js",
       "src/data/trafficQueue.js",
       "src/data/trailRenderer.js",
+      "src/data/trailContinuity.js",
       "src/data/vesselLabels.js",
       "src/data/vesselTypes.js",
       "src/firstRunExperience.js",
@@ -3631,6 +3635,7 @@
       "src/data/trafficPresetStyle.js",
       "src/data/trafficQueue.js",
       "src/data/trailRenderer.js",
+      "src/data/trailContinuity.js",
       "src/data/vesselLabels.js",
       "src/data/vesselTypes.js",
       "src/keylessGeocoder.js",