Detect a LiveTrack session ending the moment Garmin confirms it, not 30min later

Watched the session from the last two commits actually get saved:
Garmin updated its `end` field from the future 24h placeholder to
within a second of the last point's own time, immediately — much
faster than the recency-of-last-point fallback alone would have
noticed (that only flips after ACTIVE_WITHIN with no new points, so a
just-saved session with a last point seconds old would still read as
active for up to 30 more minutes).

Now checks end-already-in-the-past first (authoritative and near
instant when Garmin has set it), falling back to point recency only
when end still shows the future placeholder — the case an abandoned,
never-saved session never updates.

lib/maptool/livetrack_client.rb

@@ -56,16 +56,25 @@ module Maptool
     # doesn't set `end` until the activity actually stops — disproven
     # against a real still-recording session (2026-09-20): `end` was
     # already non-nil, set to exactly `start + 24h`, the share link's own
-    # expiry, not an actual end time (a genuinely ended session's `end`
-    # instead reflects its real end — ~2h after start for the 2026-09-06
-    # session this was originally, and only, verified against). Recency of
-    # the last track point is a directly-observable signal instead of
-    # trying to reverse-engineer what a given `end` value means: Garmin
-    # posts one roughly every `postTrackPointFrequency` seconds (~15,
-    # observed) while genuinely still recording, so a point older than
-    # ACTIVE_WITHIN means the activity has actually stopped (or, at worst,
-    # lost signal for a while — same "looks ended until it isn't" behavior
-    # Garmin's own LiveTrack page shows in that case).
+    # expiry, not an actual end time. Two signals now, both verified live
+    # against that same session:
+    #
+    # * `end` itself, once it's actually in the past — watched the same
+    #   session update it from that 24h placeholder to within a second of
+    #   the last point's own time, immediately after the activity was
+    #   saved on the watch. Authoritative and near-instant when present,
+    #   so this is checked first.
+    # * failing that (still showing the future placeholder — an abandoned
+    #   session never gets saved, so `end` never moves), recency of the
+    #   last track point: Garmin posts one roughly every
+    #   `postTrackPointFrequency` seconds (~15, observed) while genuinely
+    #   still recording, so one older than ACTIVE_WITHIN means signal was
+    #   lost a while ago (same "looks ended until it isn't" behavior
+    #   Garmin's own LiveTrack page shows in that case).
+    #
+    # Both must agree the session is live — `end` already in the past
+    # means stopped even if the last point is only seconds old (exactly
+    # the save-immediately-after-the-last-point case above).
     ACTIVE_WITHIN = 30 * 60 # seconds
 
     def self.summarize(result)
@@ -76,16 +85,19 @@ module Maptool
         "activity_type" => last["activityType"],
         "distance_meters" => last["totalDistanceMeters"],
         "duration_secs" => last["totalDurationSecs"],
-        "active" => active?(last["dateTime"]),
+        "active" => active?(last["dateTime"], session["end"]),
         "start" => session["start"],
         "end" => session["end"],
       }
     end
 
-    def self.active?(last_point_time)
+    def self.active?(last_point_time, session_end)
       return false unless last_point_time
 
-      Time.parse(last_point_time) > Time.now.utc - ACTIVE_WITHIN
+      now = Time.now.utc
+      return false if session_end && Time.parse(session_end) <= now
+
+      Time.parse(last_point_time) > now - ACTIVE_WITHIN
     rescue ArgumentError, TypeError
       false
     end