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/log.dart

@@ -4,14 +4,7 @@ import 'dart:io';
 enum LogLevel { trace, debug, info, warn, error }
 
 class LogRecord {
-  LogRecord({
-    required this.level,
-    required this.source,
-    required this.message,
-    required this.timestamp,
-    this.error,
-    this.stackTrace,
-  });
+  LogRecord({required this.level, required this.source, required this.message, required this.timestamp, this.error, this.stackTrace});
 
   final LogLevel level;
   final String source;
@@ -51,14 +44,7 @@ class Logger {
 
   void _emit(LogLevel level, String source, String message, {Object? error, StackTrace? stackTrace}) {
     if (level.index < minLevel.index) return;
-    final rec = LogRecord(
-      level: level,
-      source: source,
-      message: message,
-      timestamp: DateTime.now().toUtc(),
-      error: error,
-      stackTrace: stackTrace,
-    );
+    final rec = LogRecord(level: level, source: source, message: message, timestamp: DateTime.now().toUtc(), error: error, stackTrace: stackTrace);
     for (final sink in _sinks) {
       try {
         sink(rec);