feat: add C# and Dart language support

rules/dart/coding-style.md

@@ -0,0 +1,159 @@
+---
+paths:
+  - "**/*.dart"
+  - "**/pubspec.yaml"
+  - "**/analysis_options.yaml"
+---
+# Dart/Flutter Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with Dart and Flutter-specific content.
+
+## Formatting
+
+- **dart format** for all `.dart` files — enforced in CI (`dart format --set-exit-if-changed .`)
+- Line length: 80 characters (dart format default)
+- Trailing commas on multi-line argument/parameter lists to improve diffs and formatting
+
+## Immutability
+
+- Prefer `final` for local variables and `const` for compile-time constants
+- Use `const` constructors wherever all fields are `final`
+- Return unmodifiable collections from public APIs (`List.unmodifiable`, `Map.unmodifiable`)
+- Use `copyWith()` for state mutations in immutable state classes
+
+```dart
+// BAD
+var count = 0;
+List<String> items = ['a', 'b'];
+
+// GOOD
+final count = 0;
+const items = ['a', 'b'];
+```
+
+## Naming
+
+Follow Dart conventions:
+- `camelCase` for variables, parameters, and named constructors
+- `PascalCase` for classes, enums, typedefs, and extensions
+- `snake_case` for file names and library names
+- `SCREAMING_SNAKE_CASE` for constants declared with `const` at top level
+- Prefix private members with `_`
+- Extension names describe the type they extend: `StringExtensions`, not `MyHelpers`
+
+## Null Safety
+
+- Avoid `!` (bang operator) — prefer `?.`, `??`, `if (x != null)`, or Dart 3 pattern matching; reserve `!` only where a null value is a programming error and crashing is the right behaviour
+- Avoid `late` unless initialization is guaranteed before first use (prefer nullable or constructor init)
+- Use `required` for constructor parameters that must always be provided
+
+```dart
+// BAD — crashes at runtime if user is null
+final name = user!.name;
+
+// GOOD — null-aware operators
+final name = user?.name ?? 'Unknown';
+
+// GOOD — Dart 3 pattern matching (exhaustive, compiler-checked)
+final name = switch (user) {
+  User(:final name) => name,
+  null => 'Unknown',
+};
+
+// GOOD — early-return null guard
+String getUserName(User? user) {
+  if (user == null) return 'Unknown';
+  return user.name; // promoted to non-null after the guard
+}
+```
+
+## Sealed Types and Pattern Matching (Dart 3+)
+
+Use sealed classes to model closed state hierarchies:
+
+```dart
+sealed class AsyncState<T> {
+  const AsyncState();
+}
+
+final class Loading<T> extends AsyncState<T> {
+  const Loading();
+}
+
+final class Success<T> extends AsyncState<T> {
+  const Success(this.data);
+  final T data;
+}
+
+final class Failure<T> extends AsyncState<T> {
+  const Failure(this.error);
+  final Object error;
+}
+```
+
+Always use exhaustive `switch` with sealed types — no default/wildcard:
+
+```dart
+// BAD
+if (state is Loading) { ... }
+
+// GOOD
+return switch (state) {
+  Loading() => const CircularProgressIndicator(),
+  Success(:final data) => DataWidget(data),
+  Failure(:final error) => ErrorWidget(error.toString()),
+};
+```
+
+## Error Handling
+
+- Specify exception types in `on` clauses — never use bare `catch (e)`
+- Never catch `Error` subtypes — they indicate programming bugs
+- Use `Result`-style types or sealed classes for recoverable errors
+- Avoid using exceptions for control flow
+
+```dart
+// BAD
+try {
+  await fetchUser();
+} catch (e) {
+  log(e.toString());
+}
+
+// GOOD
+try {
+  await fetchUser();
+} on NetworkException catch (e) {
+  log('Network error: ${e.message}');
+} on NotFoundException {
+  handleNotFound();
+}
+```
+
+## Async / Futures
+
+- Always `await` Futures or explicitly call `unawaited()` to signal intentional fire-and-forget
+- Never mark a function `async` if it never `await`s anything
+- Use `Future.wait` / `Future.any` for concurrent operations
+- Check `context.mounted` before using `BuildContext` after any `await` (Flutter 3.7+)
+
+```dart
+// BAD — ignoring Future
+fetchData(); // fire-and-forget without marking intent
+
+// GOOD
+unawaited(fetchData()); // explicit fire-and-forget
+await fetchData();      // or properly awaited
+```
+
+## Imports
+
+- Use `package:` imports throughout — never relative imports (`../`) for cross-feature or cross-layer code
+- Order: `dart:` → external `package:` → internal `package:` (same package)
+- No unused imports — `dart analyze` enforces this with `unused_import`
+
+## Code Generation
+
+- Generated files (`.g.dart`, `.freezed.dart`, `.gr.dart`) must be committed or gitignored consistently — pick one strategy per project
+- Never manually edit generated files
+- Keep generator annotations (`@JsonSerializable`, `@freezed`, `@riverpod`, etc.) on the canonical source file only