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.

test/widget_test.dart

@@ -593,5 +593,65 @@ void main() {
         expect(find.text(imperialValue), findsWidgets);
       }
     });
+
+    screenTest('a single-segment ride explains why it cannot be split (V3-10)',
+        (tester) async {
+      final id = await seedCompletedTrip(startedAt: 1000, endedAt: 5000);
+
+      await tester.pumpWidget(host(TripDetailScreen(tripId: id), map: false));
+      await tester.pumpAndSettle();
+
+      await tester.tap(find.byKey(const Key('split')));
+      await tester.pumpAndSettle();
+
+      expect(find.textContaining('nothing to split'), findsOneWidget);
+      expect(await repo.segmentsForTrip(id), hasLength(1),
+          reason: 'a rejected split must not touch anything');
+    });
+
+    screenTest('splitting a multi-segment ride creates a second ride (V3-10)',
+        (tester) async {
+      final h = await repo.startTrip(1000);
+      await repo.appendPoints([
+        TrackPoint(
+          tripId: h.tripId,
+          segmentId: h.segmentId,
+          timestamp: 1000,
+          latitude: 51.0,
+          longitude: -114.0,
+          speedKmh: 40.0,
+          altitudeM: 1000.0,
+        ),
+      ]);
+      await repo.pauseTrip(2000);
+      final h2 = (await repo.resumeTrip(3000))!;
+      await repo.appendPoints([
+        TrackPoint(
+          tripId: h2.tripId,
+          segmentId: h2.segmentId,
+          timestamp: 3000,
+          latitude: 52.0,
+          longitude: -114.0,
+          speedKmh: 40.0,
+          altitudeM: 1000.0,
+        ),
+      ]);
+      await repo.completeTrip(4000);
+      final segments = await repo.segmentsForTrip(h.tripId);
+      expect(segments, hasLength(2));
+
+      await tester.pumpWidget(host(TripDetailScreen(tripId: h.tripId), map: false));
+      await tester.pumpAndSettle();
+
+      await tester.tap(find.byKey(const Key('split')));
+      await tester.pumpAndSettle();
+      await tester.tap(find.byKey(Key('split-at-${segments[1].id}')));
+      await tester.pumpAndSettle();
+      await tester.tap(find.text('Split'));
+      await tester.pumpAndSettle();
+
+      expect(await db.countTrips(), 2);
+      expect(await repo.segmentsForTrip(h.tripId), hasLength(1));
+    });
   });
 }