FB-08: reflow HUD row widgets when a metric is hidden or shown

setVisible now calls a new _reflowRow helper on the toggled metric's row,
spreading the remaining (or newly added) visible widgets evenly across the
4-column grid instead of leaving a blank cell where a hidden metric used to
be. Reflow is scoped to setVisible only; drag/resize behavior is unchanged.

test/hud_layout_controller_test.dart

@@ -110,6 +110,80 @@ void main() {
       }
     });
 
+    test('hiding one of four same-row metrics reflows the remaining three to fill '
+        'the row with no gap', () async {
+      final controller = HudLayoutController(await freshConfig());
+
+      controller.setVisible(HudMetric.avgSpeed, false);
+
+      final remaining = [HudMetric.speed, HudMetric.distance, HudMetric.elapsedTime]
+          .map((m) => controller.state[m]!)
+          .toList()
+        ..sort((a, b) => a.col.compareTo(b.col));
+      final totalSpan = remaining.fold<int>(0, (sum, l) => sum + l.colSpan);
+      expect(totalSpan, hudGridColumns);
+      // baseSpan = 4 ~/ 3 = 1, extra = 1 -- the first (leftmost) member absorbs it.
+      expect(remaining[0].colSpan, 2);
+      expect(remaining[1].colSpan, 1);
+      expect(remaining[2].colSpan, 1);
+      expect(controller.state[HudMetric.avgSpeed]!.visible, isFalse);
+    });
+
+    test('hiding a metric whose row has no other visible member leaves every '
+        'other metric untouched', () async {
+      final controller = HudLayoutController(await freshConfig());
+      // Row 4 is unused by any default layout -- move speed there alone.
+      controller.updatePosition(HudMetric.speed, 0, 4);
+
+      final others = {
+        for (final m in HudMetric.values.where((m) => m != HudMetric.speed))
+          m: controller.state[m]!,
+      };
+
+      controller.setVisible(HudMetric.speed, false);
+
+      for (final entry in others.entries) {
+        final after = controller.state[entry.key]!;
+        expect(after.col, entry.value.col, reason: '${entry.key} col changed');
+        expect(after.row, entry.value.row, reason: '${entry.key} row changed');
+        expect(after.colSpan, entry.value.colSpan,
+            reason: '${entry.key} colSpan changed');
+      }
+    });
+
+    test('showing a previously-hidden metric back into a row with existing '
+        'members reflows that row to fit it', () async {
+      final controller = HudLayoutController(await freshConfig());
+
+      // maxSpeed and movingTime both default to row 2, hidden, at adjacent columns.
+      controller.setVisible(HudMetric.maxSpeed, true);
+      controller.setVisible(HudMetric.movingTime, true);
+
+      final maxSpeed = controller.state[HudMetric.maxSpeed]!;
+      final movingTime = controller.state[HudMetric.movingTime]!;
+      expect(maxSpeed.row, movingTime.row,
+          reason: 'test setup assumes both land in the same row');
+      expect(maxSpeed.colSpan + movingTime.colSpan, hudGridColumns);
+    });
+
+    test('toggling a metric in one row leaves a different row\'s members '
+        'untouched', () async {
+      final controller = HudLayoutController(await freshConfig());
+      final row2Before = {
+        for (final m in [HudMetric.maxSpeed, HudMetric.movingTime, HudMetric.elevationGain])
+          m: controller.state[m]!,
+      };
+
+      controller.setVisible(HudMetric.avgSpeed, false);
+
+      for (final entry in row2Before.entries) {
+        final after = controller.state[entry.key]!;
+        expect(after.col, entry.value.col, reason: '${entry.key} col changed');
+        expect(after.colSpan, entry.value.colSpan,
+            reason: '${entry.key} colSpan changed');
+      }
+    });
+
     test('persist writes the current state to Config', () async {
       final config = await freshConfig();
       final controller = HudLayoutController(config);