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>

test/kernel/src/toolchain_paths_test.dart

@@ -9,13 +9,9 @@ import 'package:test/test.dart';
 void main() {
   group('ToolchainView.resolved', () {
     test('exposes the supplied paths verbatim', () {
-      final v = ToolchainView.resolved(const ResolvedPaths(
-        git: '/opt/git',
-        pql: '/opt/pql',
-        tmux: '/opt/tmux',
-        shell: '/usr/bin/zsh',
-        gitEnv: {'GIT_EXEC_PATH': '/opt/git-core'},
-      ));
+      final v = ToolchainView.resolved(
+        const ResolvedPaths(git: '/opt/git', pql: '/opt/pql', tmux: '/opt/tmux', shell: '/usr/bin/zsh', gitEnv: {'GIT_EXEC_PATH': '/opt/git-core'}),
+      );
       expect(v.git, '/opt/git');
       expect(v.pql, '/opt/pql');
       expect(v.tmux, '/opt/tmux');
@@ -39,10 +35,12 @@ void main() {
     });
 
     test('missing reports only the unresolved tools', () {
-      final v = ToolchainView.resolved(const ResolvedPaths(
-        git: '/opt/git',
-        // pql + tmux null → missing.
-      ));
+      final v = ToolchainView.resolved(
+        const ResolvedPaths(
+          git: '/opt/git',
+          // pql + tmux null → missing.
+        ),
+      );
       expect(v.missing, ['pql', 'tmux']);
       expect(v.allOk, isFalse);
     });