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_layout_controller.dart

@@ -0,0 +1,51 @@
+/// UI-04: the in-memory, authoritative HUD layout during an editing session.
+///
+/// Seeded from `Config` once, and written back to it only on [persist] -- not on every
+/// drag/resize frame, which would hammer `SharedPreferences` mid-drag (the ticket's own
+/// named risk). A drag in progress only ever touches this in-memory state; [persist] is
+/// what a `HudEditOverlay` calls when the rider exits edit mode.
+library;
+
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../config/config.dart';
+import 'hud_metric.dart';
+import 'hud_widget_layout.dart';
+
+class HudLayoutController extends StateNotifier<Map<HudMetric, HudWidgetLayout>> {
+  // `Config?`, not `Config` -- at app boot `configProvider` is briefly null (loaded
+  // asynchronously post-first-frame, same as every other Config-seeded provider in
+  // this app). Defaults stand in until it resolves; [persist] silently no-ops rather
+  // than blocking, since editing the HUD before Config has loaded isn't a real path a
+  // rider can reach in practice.
+  HudLayoutController(this._config)
+    : super(
+        _config?.hudLayout ??
+            {for (final m in HudMetric.values) m: HudWidgetLayout.defaultFor(m)},
+      );
+
+  final Config? _config;
+
+  void updatePosition(HudMetric metric, double x, double y) {
+    final current = state[metric];
+    if (current == null) return;
+    state = {...state, metric: current.copyWith(x: x, y: y).clamped()};
+  }
+
+  void updateSize(HudMetric metric, double width, double height) {
+    final current = state[metric];
+    if (current == null) return;
+    state = {...state, metric: current.copyWith(width: width, height: height).clamped()};
+  }
+
+  /// A metric turned on for the first time (no meaningfully-placed prior layout) gets
+  /// [HudWidgetLayout.defaultFor] rather than whatever stale position it held from
+  /// before it was last turned off -- the ticket's own acceptance criterion. A metric
+  /// that already has a real saved position keeps it.
+  void setVisible(HudMetric metric, bool visible) {
+    final current = state[metric] ?? HudWidgetLayout.defaultFor(metric);
+    state = {...state, metric: current.copyWith(visible: visible)};
+  }
+
+  Future<void> persist() async => _config?.setHudLayout(state);
+}