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_layout_controller_test.dart

@@ -114,6 +114,22 @@ void main() {
         'the row with no gap', () async {
       final controller = HudLayoutController(await freshConfig());
 
+      // FB-09's title-driven defaultFor no longer puts all four default-visible
+      // metrics in row 0 with uniform colSpan 1 (avgSpeed and elapsedTime need 2
+      // columns for their longer titles). Force that uniform row-0 arrangement
+      // explicitly here so this test exercises _reflowRow's own algorithm in
+      // isolation, independent of exactly where defaultFor happens to place things.
+      controller.updateSize(HudMetric.elapsedTime, 1, 2);
+      controller.updateSize(HudMetric.avgSpeed, 1, 2);
+      controller.updatePosition(HudMetric.elapsedTime, 2, 0);
+      expect(
+        [HudMetric.speed, HudMetric.avgSpeed, HudMetric.elapsedTime, HudMetric.distance]
+            .map((m) => controller.state[m]!.row)
+            .toSet(),
+        {0},
+        reason: 'test setup assumes all four land in row 0',
+      );
+
       controller.setVisible(HudMetric.avgSpeed, false);
 
       final remaining = [HudMetric.speed, HudMetric.distance, HudMetric.elapsedTime]
@@ -155,7 +171,14 @@ void main() {
         '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.
+      // FB-09's title-driven defaultFor no longer necessarily lands maxSpeed and
+      // movingTime in the same row (their default rows depend on every metric's
+      // individual title width). Move both hidden metrics into the same row
+      // explicitly, before enabling either, so this test is independent of exactly
+      // where defaultFor happens to place them.
+      controller.updatePosition(HudMetric.maxSpeed, 0, 6);
+      controller.updatePosition(HudMetric.movingTime, 1, 6);
+
       controller.setVisible(HudMetric.maxSpeed, true);
       controller.setVisible(HudMetric.movingTime, true);