chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/kernel/src/panels/arrangement.dart

@@ -19,13 +19,7 @@ class LayoutArrangement extends ChangeNotifier {
     _focusModeSnapshot = null;
     _focusModeSlot = null;
     for (final slot in preset.slots) {
-      _state[slot.slot] = _SlotState(
-        position: slot.position,
-        size: slot.defaultSize,
-        minSize: slot.minSize,
-        maxSize: slot.maxSize,
-        visible: slot.visible,
-      );
+      _state[slot.slot] = _SlotState(position: slot.position, size: slot.defaultSize, minSize: slot.minSize, maxSize: slot.maxSize, visible: slot.visible);
     }
     notifyListeners();
   }
@@ -131,26 +125,15 @@ class LayoutArrangement extends ChangeNotifier {
 
   void registerSlotsInto(PanelRegistry registry, LayoutPresetContribution preset) {
     for (final slot in preset.slots) {
-      registry.registerSlot(SlotDefinition(
-        id: slot.slot,
-        position: slot.position,
-        defaultSize: slot.defaultSize,
-        minSize: slot.minSize,
-        maxSize: slot.maxSize,
-      ));
+      registry.registerSlot(
+        SlotDefinition(id: slot.slot, position: slot.position, defaultSize: slot.defaultSize, minSize: slot.minSize, maxSize: slot.maxSize),
+      );
     }
   }
 }
 
 class _SlotState {
-  const _SlotState({
-    required this.position,
-    this.size,
-    this.minSize,
-    this.maxSize,
-    this.visible = true,
-    this.collapsed = false,
-  });
+  const _SlotState({required this.position, this.size, this.minSize, this.maxSize, this.visible = true, this.collapsed = false});
 
   final SlotPosition position;
   final double? size;
@@ -159,14 +142,7 @@ class _SlotState {
   final bool visible;
   final bool collapsed;
 
-  _SlotState copyWith({
-    SlotPosition? position,
-    double? size,
-    double? minSize,
-    double? maxSize,
-    bool? visible,
-    bool? collapsed,
-  }) {
+  _SlotState copyWith({SlotPosition? position, double? size, double? minSize, double? maxSize, bool? visible, bool? collapsed}) {
     return _SlotState(
       position: position ?? this.position,
       size: size ?? this.size,

lib/kernel/src/panels/drag_resize.dart

@@ -12,13 +12,7 @@ import 'package:flutter/widgets.dart';
 /// coarse step). Exposes a `slider` Semantics node so screen readers
 /// announce the current width.
 class DragResizeHandle extends StatefulWidget {
-  const DragResizeHandle({
-    super.key,
-    required this.arrangement,
-    required this.slot,
-    required this.axis,
-    this.thickness = DragResizeHandle.defaultThickness,
-  });
+  const DragResizeHandle({super.key, required this.arrangement, required this.slot, required this.axis, this.thickness = DragResizeHandle.defaultThickness});
 
   final LayoutArrangement arrangement;
   final SlotId slot;
@@ -155,11 +149,7 @@ class _BumpIntent extends Intent {
 /// delta = right/down. Context-panel sits on the right edge of the
 /// app, so we flip the sign there — right-arrow should *shrink* it,
 /// matching how dragging the left-edge handle rightward works.
-double bumpedSlotSize({
-  required SlotId slot,
-  required double current,
-  required double rawDelta,
-}) {
+double bumpedSlotSize({required SlotId slot, required double current, required double rawDelta}) {
   final delta = slot == Slots.contextPanel ? -rawDelta : rawDelta;
   return current + delta;
 }

lib/kernel/src/panels/layout_preset.dart

@@ -11,41 +11,13 @@ import 'package:clide/kernel/src/panels/slot_id.dart';
 ///   context  420 (drag 220–420)
 ///   statusbar 26 (fixed height strip)
 LayoutPresetContribution classicPreset() => const LayoutPresetContribution(
-      id: 'builtin.default-layout.classic',
-      displayName: 'Classic',
-      slots: [
-        LayoutSlot(
-          slot: Slots.sidebar,
-          position: SlotPosition.left,
-          defaultSize: 400,
-          minSize: 180,
-          maxSize: 400,
-        ),
-        LayoutSlot(
-          slot: Slots.workspace,
-          position: SlotPosition.center,
-        ),
-        LayoutSlot(
-          slot: Slots.contextPanel,
-          position: SlotPosition.right,
-          defaultSize: 420,
-          minSize: 220,
-          maxSize: 1000,
-        ),
-        LayoutSlot(
-          slot: Slots.dock,
-          position: SlotPosition.bottom,
-          defaultSize: 200,
-          minSize: 100,
-          maxSize: 600,
-          visible: false,
-        ),
-        LayoutSlot(
-          slot: Slots.statusbar,
-          position: SlotPosition.bottom,
-          defaultSize: 26,
-          minSize: 26,
-          maxSize: 26,
-        ),
-      ],
-    );
+  id: 'builtin.default-layout.classic',
+  displayName: 'Classic',
+  slots: [
+    LayoutSlot(slot: Slots.sidebar, position: SlotPosition.left, defaultSize: 400, minSize: 180, maxSize: 400),
+    LayoutSlot(slot: Slots.workspace, position: SlotPosition.center),
+    LayoutSlot(slot: Slots.contextPanel, position: SlotPosition.right, defaultSize: 420, minSize: 220, maxSize: 1000),
+    LayoutSlot(slot: Slots.dock, position: SlotPosition.bottom, defaultSize: 200, minSize: 100, maxSize: 600, visible: false),
+    LayoutSlot(slot: Slots.statusbar, position: SlotPosition.bottom, defaultSize: 26, minSize: 26, maxSize: 26),
+  ],
+);

lib/kernel/src/panels/registry.dart

@@ -4,13 +4,7 @@ import 'package:flutter/foundation.dart';
 
 @immutable
 class SlotDefinition {
-  const SlotDefinition({
-    required this.id,
-    required this.position,
-    this.defaultSize,
-    this.minSize,
-    this.maxSize,
-  });
+  const SlotDefinition({required this.id, required this.position, this.defaultSize, this.minSize, this.maxSize});
 
   final SlotId id;
   final SlotPosition position;

lib/kernel/src/panels/view_pane_snapshot.dart

@@ -20,13 +20,7 @@ List<ViewPane> snapshotViewPanes(PanelRegistry panels, LayoutArrangement arrange
     final activeId = panels.activeTabIn(slot.id);
     final visible = arrangement.isVisible(slot.id);
     for (final tab in panels.tabsFor(slot.id)) {
-      out.add(ViewPane(
-        id: tab.id,
-        slot: slot.id.value,
-        title: tab.title,
-        active: tab.id == activeId,
-        visible: visible,
-      ));
+      out.add(ViewPane(id: tab.id, slot: slot.id.value, title: tab.title, active: tab.id == activeId, visible: visible));
     }
   }
   return out;