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.

lib/src/hud/hud_layout_controller.dart

@@ -67,7 +67,30 @@ class HudLayoutController extends StateNotifier<Map<HudMetric, HudWidgetLayout>>
         rowSpan: current.rowSpan,
       );
     }
-    state = {...state, metric: current.copyWith(visible: visible)};
+    final updated = {...state, metric: current.copyWith(visible: visible)};
+    state = _reflowRow(updated, current.row);
+  }
+
+  /// Spreads every visible widget in [row] evenly across the full grid width,
+  /// left to right in their existing column order. A widget whose row has no other
+  /// visible member is left alone -- there is nothing to redistribute.
+  Map<HudMetric, HudWidgetLayout> _reflowRow(
+    Map<HudMetric, HudWidgetLayout> layouts,
+    int row,
+  ) {
+    final members = layouts.values.where((l) => l.visible && l.row == row).toList()
+      ..sort((a, b) => a.col.compareTo(b.col));
+    if (members.length < 2) return layouts;
+    final baseSpan = hudGridColumns ~/ members.length;
+    final extra = hudGridColumns % members.length;
+    final result = Map<HudMetric, HudWidgetLayout>.from(layouts);
+    var col = 0;
+    for (var i = 0; i < members.length; i++) {
+      final span = baseSpan + (i < extra ? 1 : 0);
+      result[members[i].metric] = members[i].copyWith(col: col, colSpan: span);
+      col += span;
+    }
+    return result;
   }
 
   Future<void> persist() async => _config?.setHudLayout(state);