FB-09: size default HUD widgets by title, not a fixed uniform cell

Adds minColSpanForLabel as a character-count proxy for how many grid
columns a metric's title needs, reworks HudWidgetLayout.defaultFor into
a left-to-right row-packing layout using it, enforces the same minimum
as a floor on manual resize, and splits _HudMetricValue's FittedBox so
the title renders at a fixed reference size while only the value
shrinks/grows to fill the remaining space.

Confirms a real, documented interaction with FB-08's _reflowRow: its
even column split has no awareness of a member's title-driven minimum
and can push one below it. Left unfixed per this ticket's own
Out-of-scope section -- _reflowRow's algorithm is FB-08's, not this
ticket's, to change.

Test count 426 -> 434, all green. flutter analyze unchanged (same 4
pre-existing, unrelated info-level issues).

lib/src/ui/components/draggable_resizable_hud_widget.dart

@@ -146,9 +146,18 @@ class _DraggableResizableHudWidgetState extends State<DraggableResizableHudWidge
                   });
                 },
                 onPanEnd: (_) {
+                  // FB-09: a rider can still make a widget larger than its title
+                  // needs (for a bigger value reading), but never smaller than the
+                  // title's own minimum -- clamped here rather than left to the
+                  // generic hudMinSpan floor in HudLayoutController.
                   final snappedColSpan =
-                      ((layout.colSpan * _cellWidth + _resizeDelta.width) / _cellWidth)
-                          .round();
+                      (((layout.colSpan * _cellWidth + _resizeDelta.width) /
+                              _cellWidth)
+                          .round())
+                          .clamp(
+                            minColSpanForLabel(layout.metric.label),
+                            hudMaxColSpan,
+                          );
                   final snappedRowSpan =
                       ((layout.rowSpan * _cellHeight + _resizeDelta.height) /
                               _cellHeight)