UI-04: customizable, persisted HUD telemetry widget layout

Adds the full drag/resize/visibility infrastructure for a rider-owned HUD
layout: HudMetric (the stable 8-metric set), HudWidgetLayout (fractional
x/y/width/height + visible, with clamping to a legibility floor/ceiling
and JSON round-trip that degrades to sane defaults rather than crashing),
Config.hudLayout persistence, and a HudLayoutController that stays
in-memory-authoritative during an edit session and only writes through on
persist() -- never per drag frame.

DraggableResizableHudWidget and HudEditOverlay assemble the interaction:
edit mode is entered by a long-press on empty HUD space (not a specific
widget) and exited via Done or a tap on empty space; only while editing
does a widget attach any drag/resize gesture at all, so a normal tap can
never move one mid-ride by construction, not by an internal flag.

Fixes a real gesture-arena bug found during testing: outside edit mode, a
long-press landing on a widget was free to bubble to the overlay's
background long-press handler and wrongly enter edit mode. An inner no-op
GestureDetector of the same gesture type now absorbs it.

Adds a "Live HUD stats" Settings section, one switch per metric, that
persists immediately (unlike drag frames). Placed at the end of the
Settings list rather than in the middle -- inserting mid-list pushed
every later section below several existing tests' viewport assumptions.

Verified end-to-end on a real emulator including a full process restart:
resized a widget, force-stopped the app, relaunched, and the resize held.
No consuming screen exists yet (UI-05's job) -- verified via a throwaway
preview entry point, deleted after use.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

test/config_test.dart

@@ -1,6 +1,8 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:rippr/src/config/config.dart';
 import 'package:rippr/src/domain/models.dart';
+import 'package:rippr/src/hud/hud_metric.dart';
+import 'package:rippr/src/hud/hud_widget_layout.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 /// `Config` had no dedicated tests before V3-03 — every existing widget test leaves
@@ -81,6 +83,57 @@ void main() {
     });
   });
 
+  group('hudLayout (UI-04)', () {
+    test('a fresh install gets a default layout for every metric', () async {
+      final config = await freshConfig();
+      final layout = config.hudLayout;
+      expect(layout.keys.toSet(), HudMetric.values.toSet());
+    });
+
+    test('round-trips an edited layout through set/get', () async {
+      final config = await freshConfig();
+      final edited = {
+        ...config.hudLayout,
+        HudMetric.speed: const HudWidgetLayout(
+          metric: HudMetric.speed,
+          x: 0.33,
+          y: 0.44,
+          width: 0.25,
+          height: 0.1,
+          visible: true,
+        ),
+      };
+
+      await config.setHudLayout(edited);
+      final reloaded = config.hudLayout;
+
+      expect(reloaded[HudMetric.speed]!.x, 0.33);
+      expect(reloaded[HudMetric.speed]!.y, 0.44);
+    });
+
+    test('a metric missing from a saved layout (e.g. one added in a later app '
+        'version) falls back to its default rather than being omitted', () async {
+      SharedPreferences.setMockInitialValues({
+        'hud_layout': '{"speed": {"x": 0.1, "y": 0.1, "width": 0.2, "height": 0.1, '
+            '"visible": true}}',
+      });
+      final config = await Config.load();
+      final layout = config.hudLayout;
+
+      expect(layout.keys.toSet(), HudMetric.values.toSet());
+      final expectedDefault = HudWidgetLayout.defaultFor(HudMetric.distance);
+      expect(layout[HudMetric.distance]!.x, expectedDefault.x);
+      expect(layout[HudMetric.distance]!.visible, expectedDefault.visible);
+    });
+
+    test('corrupted stored JSON falls back to defaults rather than crashing',
+        () async {
+      SharedPreferences.setMockInitialValues({'hud_layout': 'not json at all'});
+      final config = await Config.load();
+      expect(config.hudLayout.keys.toSet(), HudMetric.values.toSet());
+    });
+  });
+
   group('deviceId', () {
     test('is generated once and then stable across reads', () async {
       final config = await freshConfig();