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

lib/src/hud/hud_widget_layout.dart

@@ -0,0 +1,121 @@
+/// UI-04: one telemetry widget's position, size, and visibility on the customizable
+/// HUD -- see the ticket's Design section for why these are fractions (0.0-1.0) of the
+/// available HUD area rather than absolute pixels: a layout saved on one device or
+/// orientation still makes sense on another.
+library;
+
+import 'hud_metric.dart';
+
+/// Legibility floor and a sane ceiling -- a widget must never shrink to the point its
+/// own number is unreadable, or grow to the point it swallows the whole HUD.
+const double hudMinWidthFraction = 0.20;
+const double hudMaxWidthFraction = 0.70;
+const double hudMinHeightFraction = 0.08;
+const double hudMaxHeightFraction = 0.40;
+
+class HudWidgetLayout {
+  const HudWidgetLayout({
+    required this.metric,
+    required this.x,
+    required this.y,
+    required this.width,
+    required this.height,
+    required this.visible,
+  });
+
+  final HudMetric metric;
+
+  /// Top-left corner, as a fraction of the HUD area's width/height.
+  final double x;
+  final double y;
+  final double width;
+  final double height;
+  final bool visible;
+
+  HudWidgetLayout copyWith({
+    double? x,
+    double? y,
+    double? width,
+    double? height,
+    bool? visible,
+  }) => HudWidgetLayout(
+    metric: metric,
+    x: x ?? this.x,
+    y: y ?? this.y,
+    width: width ?? this.width,
+    height: height ?? this.height,
+    visible: visible ?? this.visible,
+  );
+
+  /// Corrects a drag/resize result that ended outside the allowed area back to the
+  /// nearest valid position/size -- clamped to the [hudMinWidthFraction]/
+  /// [hudMaxWidthFraction] etc. bounds first (size), then positioned so it can never
+  /// sit even partially outside the 0.0-1.0 HUD area (position), in that order: a
+  /// resize that would push a widget off-screen should shrink it back on-screen, not
+  /// silently reposition it out from under the rider's finger.
+  HudWidgetLayout clamped() {
+    final clampedWidth = width.clamp(hudMinWidthFraction, hudMaxWidthFraction);
+    final clampedHeight = height.clamp(hudMinHeightFraction, hudMaxHeightFraction);
+    final clampedX = x.clamp(0.0, 1.0 - clampedWidth);
+    final clampedY = y.clamp(0.0, 1.0 - clampedHeight);
+    return HudWidgetLayout(
+      metric: metric,
+      x: clampedX,
+      y: clampedY,
+      width: clampedWidth,
+      height: clampedHeight,
+      visible: visible,
+    );
+  }
+
+  Map<String, dynamic> toJson() => {
+    'x': x,
+    'y': y,
+    'width': width,
+    'height': height,
+    'visible': visible,
+  };
+
+  /// Falls back to [defaultFor] rather than throwing on a malformed/partial entry --
+  /// an old saved layout from a future app version with fields this version doesn't
+  /// recognise should degrade to a sane default, not crash Settings on launch.
+  static HudWidgetLayout fromJson(HudMetric metric, Map<String, dynamic> json) {
+    try {
+      return HudWidgetLayout(
+        metric: metric,
+        x: (json['x'] as num).toDouble(),
+        y: (json['y'] as num).toDouble(),
+        width: (json['width'] as num).toDouble(),
+        height: (json['height'] as num).toDouble(),
+        visible: json['visible'] as bool,
+      ).clamped();
+    } catch (_) {
+      return HudWidgetLayout.defaultFor(metric);
+    }
+  }
+
+  /// A deterministic starting grid -- a fresh install has a working, if plain, HUD
+  /// before the rider customises anything, and a metric toggled on for the first time
+  /// (with no saved position) lands somewhere sane rather than stacked on another
+  /// widget. Two rows of four, matching the Stitch mockup's row of cards for however
+  /// many metrics fit in the first row, with the rest continuing below it.
+  factory HudWidgetLayout.defaultFor(HudMetric metric) {
+    const columns = 4;
+    const cellWidth = 0.22;
+    const cellHeight = 0.12;
+    const gap = 0.02;
+    final index = HudMetric.values.indexOf(metric);
+    final row = index ~/ columns;
+    final col = index % columns;
+    return HudWidgetLayout(
+      metric: metric,
+      x: 0.02 + col * (cellWidth + gap),
+      y: 0.06 + row * (cellHeight + gap),
+      width: cellWidth,
+      height: cellHeight,
+      // The mockup's own fixed row is Speed/Distance/Elapsed/Max speed -- the first
+      // four enum values are ordered to match, so only those start visible.
+      visible: index < columns,
+    );
+  }
+}