docs: primary constructor tip and correct minimum SDK

site/src/content/docs/concepts.md

@@ -29,6 +29,12 @@ final class Product extends AppRoute {
 }
 ```
 
+:::tip[On Dart 3.13 or later?]
+Primary constructors shrink a variant to its header:
+`final class const Home() extends AppRoute;`. See
+[Getting started](/getting-started/) for the full form.
+:::
+
 Two consequences fall out immediately. Routes have **equality**
 (`Product('42') == Product('42')`), so the router can diff, dedupe, and
 restore them. And routes are **exhaustively matchable** — a `switch` over

site/src/content/docs/getting-started.md

@@ -13,7 +13,7 @@ your screens are an exhaustive `switch`, and the navigation stack is a plain
 flutter pub add kaisel
 ```
 
-kaisel supports Flutter 3.24+ and requires Dart 3.
+kaisel requires Dart 3.10 or later, which ships with Flutter 3.38.
 
 ## 1. Declare your route type
 
@@ -36,6 +36,23 @@ final class ProductDetail extends AppRoute {
 No-field variants get equality automatically (`Home() == Home()`). Variants
 with fields override `props`.
 
+:::tip[On Dart 3.13 or later?]
+Primary constructors fold the constructor and its fields into the class
+header. If your package's SDK constraint is `^3.13.0` or higher, the same
+routes can read:
+
+```dart
+final class const Home() extends AppRoute;
+
+final class const ProductDetail(final String id) extends AppRoute {
+  @override
+  List<Object?> get props => [id];
+}
+```
+
+kaisel needs nothing extra for this. Equality still comes from `props`.
+:::
+
 ## 2. Wire it up
 
 Bundle the router into a `KaiselRouterConfig` and hand it to

skills/kaisel/SKILL.md

@@ -122,6 +122,28 @@ final class ProductDetail extends AppRoute {
   unequal and the stack will treat them as distinct entries.
 - Sealed type at the root. Pattern matching downstream depends on
   exhaustiveness, which requires the base to be `sealed`.
+- Match the project's language version. Check the `environment: sdk:`
+  lower bound in the user's `pubspec.yaml`. At `^3.13.0` or higher, write
+  route variants with primary constructors; below that they fail to
+  compile, so use the classic form above. Don't mix the two forms in one
+  route file, and don't raise the user's SDK constraint just to use them.
+
+  ```dart
+  // sdk: ^3.13.0 or higher
+  final class const Home() extends AppRoute;
+
+  final class const ProductList({final String? category}) extends AppRoute {
+    @override
+    List<Object?> get props => [category];
+  }
+
+  final class const ProductDetail(final String id) extends AppRoute {
+    @override
+    List<Object?> get props => [id];
+  }
+  ```
+
+  Primary constructors don't generate `==`, so `props` is still required.
 
 ## 2. Wiring up the router