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_test.dart

@@ -24,13 +24,15 @@ void main() {
       final t = Toolchain();
       var calls = 0;
       t.addListener(() => calls++);
-      t.applyResolved(const ResolvedPaths(
-        git: '/usr/bin/git',
-        pql: '/usr/bin/pql',
-        tmux: '/usr/bin/tmux',
-        shell: '/bin/bash',
-        gitEnv: {'GIT_EXEC_PATH': '/usr/lib/git-core'},
-      ));
+      t.applyResolved(
+        const ResolvedPaths(
+          git: '/usr/bin/git',
+          pql: '/usr/bin/pql',
+          tmux: '/usr/bin/tmux',
+          shell: '/bin/bash',
+          gitEnv: {'GIT_EXEC_PATH': '/usr/lib/git-core'},
+        ),
+      );
       expect(t.resolved, isTrue);
       expect(t.allOk, isTrue);
       expect(t.missing, isEmpty);