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/keymap/key_chord.dart

@@ -23,11 +23,11 @@ enum KeyModifier {
 
   /// Display string used in palette + tooltip hints.
   String get display => switch (this) {
-        KeyModifier.ctrl => 'Ctrl',
-        KeyModifier.alt => 'Alt',
-        KeyModifier.shift => 'Shift',
-        KeyModifier.meta => 'Cmd',
-      };
+    KeyModifier.ctrl => 'Ctrl',
+    KeyModifier.alt => 'Alt',
+    KeyModifier.shift => 'Shift',
+    KeyModifier.meta => 'Cmd',
+  };
 }
 
 /// A modifier-set + a single key, identified by layout-independent

lib/kernel/src/keymap/keymap.dart

@@ -23,11 +23,7 @@ import 'when_clause.dart';
 /// to fire when the sequence matches and the when-clause is true.
 @immutable
 class KeymapBinding {
-  const KeymapBinding({
-    required this.sequence,
-    required this.intent,
-    this.when,
-  });
+  const KeymapBinding({required this.sequence, required this.intent, this.when});
 
   /// Convenience constructor for a single-chord binding.
   KeymapBinding.chord(KeyChord chord, {required this.intent, this.when}) : sequence = [chord];
@@ -204,9 +200,7 @@ class Keymap {
   /// must come first. We don't dedupe — a no-op match in a later layer
   /// just earns the first slot.
   static List<KeymapBinding> _flatten(List<KeymapLayer> layers) {
-    return [
-      for (final l in layers.reversed) ...l.bindings,
-    ];
+    return [for (final l in layers.reversed) ...l.bindings];
   }
 
   @override

lib/kernel/src/keymap/keymap_service.dart

@@ -41,13 +41,10 @@ const String kKeymapOverridesSetting = 'app.keymap.overrides';
 const String kKeymapUserFile = 'keybindings.yaml';
 
 class KeymapService extends ChangeNotifier {
-  KeymapService({
-    required SettingsStore settings,
-    required Directory appDir,
-    AssetBundle? bundle,
-  })  : _settings = settings,
-        _appDir = appDir,
-        _bundle = bundle ?? rootBundle;
+  KeymapService({required SettingsStore settings, required Directory appDir, AssetBundle? bundle})
+    : _settings = settings,
+      _appDir = appDir,
+      _bundle = bundle ?? rootBundle;
 
   final SettingsStore _settings;
   final Directory _appDir;
@@ -127,11 +124,7 @@ class KeymapService extends ChangeNotifier {
   /// defaults, the user can override either via the user file or
   /// settings overlay.
   void registerCommandBinding(String chordSpec, String commandId, {String? when}) {
-    _contributions.add(KeymapBinding(
-      sequence: KeyChord.parseSequence(chordSpec),
-      intent: InvokeCommandIntent(commandId),
-      when: WhenExpr.tryParse(when),
-    ));
+    _contributions.add(KeymapBinding(sequence: KeyChord.parseSequence(chordSpec), intent: InvokeCommandIntent(commandId), when: WhenExpr.tryParse(when)));
     _rebuildActive();
   }
 
@@ -149,12 +142,7 @@ class KeymapService extends ChangeNotifier {
   }
 
   void _rebuildActive() {
-    final layers = <KeymapLayer>[
-      if (_preset != null) _preset!,
-      KeymapLayer(name: 'contributions', bindings: List.unmodifiable(_contributions)),
-      if (_userFile != null) _userFile!,
-      if (_settingsOverlay != null) _settingsOverlay!,
-    ];
+    final layers = <KeymapLayer>[?_preset, KeymapLayer(name: 'contributions', bindings: List.unmodifiable(_contributions)), ?_userFile, ?_settingsOverlay];
     _active = Keymap(layers);
     notifyListeners();
   }

lib/kernel/src/keymap/sequence_matcher.dart

@@ -33,18 +33,9 @@ enum SeqOutcome {
 
 @immutable
 class SeqResult {
-  const SeqResult.fired(Intent this.intent, this.count)
-      : outcome = SeqOutcome.fired,
-        passKey = null;
-  const SeqResult.pending()
-      : outcome = SeqOutcome.pending,
-        intent = null,
-        count = 1,
-        passKey = null;
-  const SeqResult.unmatched(this.passKey)
-      : outcome = SeqOutcome.unmatched,
-        intent = null,
-        count = 1;
+  const SeqResult.fired(Intent this.intent, this.count) : outcome = SeqOutcome.fired, passKey = null;
+  const SeqResult.pending() : outcome = SeqOutcome.pending, intent = null, count = 1, passKey = null;
+  const SeqResult.unmatched(this.passKey) : outcome = SeqOutcome.unmatched, intent = null, count = 1;
 
   final SeqOutcome outcome;
   final Intent? intent;
@@ -53,12 +44,9 @@ class SeqResult {
 }
 
 class SequenceMatcher {
-  SequenceMatcher({
-    required Keymap Function() keymap,
-    required Map<String, bool> Function() context,
-    this.captureCounts = true,
-  })  : _keymap = keymap,
-        _context = context;
+  SequenceMatcher({required Keymap Function() keymap, required Map<String, bool> Function() context, this.captureCounts = true})
+    : _keymap = keymap,
+      _context = context;
 
   final Keymap Function() _keymap;
   final Map<String, bool> Function() _context;