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).

test/hud_widget_layout_test.dart

@@ -89,6 +89,42 @@ void main() {
         expect(layout.clampedToGrid().rowSpan, layout.rowSpan);
       }
     });
+
+    test('every metric\'s colSpan is at least its own title-driven minimum', () {
+      for (final metric in HudMetric.values) {
+        final layout = HudWidgetLayout.defaultFor(metric);
+        expect(
+          layout.colSpan,
+          greaterThanOrEqualTo(minColSpanForLabel(metric.label)),
+        );
+      }
+    });
+
+    test('no row\'s members ever sum past hudGridColumns (the packing '
+        'algorithm\'s own invariant)', () {
+      final layouts = HudMetric.values.map(HudWidgetLayout.defaultFor).toList();
+      final byRow = <int, int>{};
+      for (final layout in layouts) {
+        byRow[layout.row] = (byRow[layout.row] ?? 0) + layout.colSpan;
+      }
+      for (final sum in byRow.values) {
+        expect(sum, lessThanOrEqualTo(hudGridColumns));
+      }
+    });
+  });
+
+  group('minColSpanForLabel', () {
+    test('a label under 10 characters fits one column', () {
+      expect(minColSpanForLabel(HudMetric.speed.label), 1);
+    });
+
+    test('a label under 15 characters needs two', () {
+      expect(minColSpanForLabel(HudMetric.elapsedTime.label), 2);
+    });
+
+    test('a label of 15 or more characters needs three', () {
+      expect(minColSpanForLabel(HudMetric.pointsCaptured.label), 3);
+    });
   });
 
   group('clampedToGrid', () {