Add view_model architecture sample

[lwj1994]

Summary

• Add a complete TodoMVC sample implemented with the view_model package.
• Resolve the shared TodoListViewModel from each screen through its own ViewModelBinding.read(spec) call, using the repository as an explicit sharing key.
• Add ViewModel unit tests, widget flow tests, the shared integration test suite, and serial test configuration.
• Register the sample in the README, website, line counter, CI, and combined coverage job.
• Reserve https://fas-view-model.netlify.app as the conventional Web Demo URL; deployment remains disabled until the Netlify site is configured.

Validation

• dart format --output=none --set-exit-if-changed .
• flutter analyze
• flutter test --coverage --concurrency=1 (7 tests, 78.21% coverage)
• flutter test integration_test/app_test.dart -d macos
• flutter build web --wasm