V3-12: crash reporting (code only)

shouldInitializeCrashReporting() and scrubExtra() are pure, fully unit-tested decision/scrubbing functions wired into sentry_flutter via beforeSend/beforeBreadcrumb. Config.crashReportingEnabled defaults off; the DSN is a compile-time --dart-define, not a preference. RecordingEngine gained an injected onUnexpectedStop callback, firing once when restoreAfterProcessDeath finds a trip still 'recording' at launch -- the process died without a user stop.

Marked partially done: the ticket's real acceptance criteria (a forced crash landing in a real Sentry project from a release build, inspecting a real payload) need a Sentry account and a release build this environment can't produce.

test/config_test.dart

@@ -55,6 +55,19 @@ void main() {
     });
   });
 
+  group('crashReportingEnabled', () {
+    test('defaults to false', () async {
+      final config = await freshConfig();
+      expect(config.crashReportingEnabled, isFalse);
+    });
+
+    test('round-trips through set/get', () async {
+      final config = await freshConfig();
+      await config.setCrashReportingEnabled(true);
+      expect(config.crashReportingEnabled, isTrue);
+    });
+  });
+
   group('uploadEndpoint', () {
     test('defaults to empty, not null or a placeholder', () async {
       final config = await freshConfig();