V3-10: trip splitting

TripRepository.splitTrip mirrors mergeTrips' transaction shape: splits at a segment boundary, moves the target segment and everything after it to a new trip, recomputes both trips' aggregates from scratch. startedAt/endedAt derive from each trip's actual remaining segments, not copied from the pre-split row. Trip detail gained a split action with a segment-boundary picker and a naming confirmation; a single-segment ride explains why it can't split instead of offering a dead control.

lib/src/data/database.dart

@@ -337,6 +337,13 @@ class AppDatabase extends _$AppDatabase {
         SegmentsCompanion(tripId: Value(newTripId)),
       );
 
+  /// Used by split (V3-10): unlike [reparentSegments], moves exactly one segment rather
+  /// than every segment on a trip.
+  Future<void> reparentSegment(int segmentId, int newTripId) =>
+      (update(segments)..where((s) => s.id.equals(segmentId))).write(
+        SegmentsCompanion(tripId: Value(newTripId)),
+      );
+
   Future<int> countSegmentsForTrip(int tripId) async => (await (select(
     segments,
   )..where((s) => s.tripId.equals(tripId))).get()).length;
@@ -499,6 +506,17 @@ class AppDatabase extends _$AppDatabase {
         TrackPointsCompanion(tripId: Value(newTripId)),
       );
 
+  /// Used by split (V3-10): moves only the points belonging to the given segments, not
+  /// every point on the trip -- the counterpart to [reparentSegment].
+  Future<void> reparentPointsForSegments(List<int> segmentIds, int newTripId) {
+    if (segmentIds.isEmpty) return Future.value();
+    return (update(
+      trackPoints,
+    )..where((p) => p.segmentId.isIn(segmentIds))).write(
+      TrackPointsCompanion(tripId: Value(newTripId)),
+    );
+  }
+
   Future<List<domain.TrackPoint>> allPoints() async {
     final rows = await (select(
       trackPoints,