feat: make breadcrumb component experimental behind feature flag

Add experimental feature flag to breadcrumb components following the
established pattern used by other experimental Vaadin components.
Components are now hidden behind the breadcrumbComponent feature flag
and must be explicitly enabled.

- Add static experimental() getter to both breadcrumb classes
- Update JSDoc and TypeScript documentation with experimental warning
- Update dev example and tests to enable feature flag
- Follow same pattern as vaadin-master-detail-layout

Users must enable with:
window.Vaadin.featureFlags.breadcrumbComponent = true

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Artur-

dev/breadcrumb.html

@@ -6,6 +6,11 @@
   <title>vaadin-breadcrumb</title>
   <script type="module" src="./common.js"></script>
   <script type="module">
+    // Enable experimental breadcrumb component
+    window.Vaadin ||= {};
+    window.Vaadin.featureFlags ||= {};
+    window.Vaadin.featureFlags.breadcrumbComponent = true;
+    
     import '@vaadin/breadcrumb/vaadin-breadcrumb.js';
     import '@vaadin/breadcrumb/vaadin-breadcrumb-item.js';
   </script>

packages/breadcrumb/src/vaadin-breadcrumb-item.d.ts

@@ -11,6 +11,8 @@ import { ThemableMixin } from '@vaadin/vaadin-themable-mixin/vaadin-themable-mix
 /**
  * `<vaadin-breadcrumb-item>` is a Web Component for displaying a single item in a breadcrumb trail.
  *
+ * ⚠️ **This component is experimental** and the API may change. In order to use it, enable the feature flag by setting `window.Vaadin.featureFlags.breadcrumbComponent = true`.
+ *
  * ```html
  * <vaadin-breadcrumb-item href="/products">Products</vaadin-breadcrumb-item>
  * ```

packages/breadcrumb/src/vaadin-breadcrumb-item.js

@@ -18,6 +18,8 @@ import { breadcrumbItemStyles } from './styles/vaadin-breadcrumb-item-styles.js'
 /**
  * `<vaadin-breadcrumb-item>` is a Web Component for displaying a single item in a breadcrumb trail.
  *
+ * ⚠️ **This component is experimental** and the API may change. In order to use it, enable the feature flag by setting `window.Vaadin.featureFlags.breadcrumbComponent = true`.
+ *
  * ```html
  * <vaadin-breadcrumb-item href="/products">Products</vaadin-breadcrumb-item>
  * ```
@@ -55,6 +57,10 @@ class BreadcrumbItem extends DisabledMixin(
     return 'vaadin-breadcrumb-item';
   }
 
+  static get experimental() {
+    return true;
+  }
+
   static get styles() {
     return breadcrumbItemStyles;
   }

packages/breadcrumb/src/vaadin-breadcrumb.d.ts

@@ -9,6 +9,8 @@ import { ThemableMixin } from '@vaadin/vaadin-themable-mixin/vaadin-themable-mix
 /**
  * `<vaadin-breadcrumb>` is a Web Component for displaying hierarchical navigation.
  *
+ * ⚠️ **This component is experimental** and the API may change. In order to use it, enable the feature flag by setting `window.Vaadin.featureFlags.breadcrumbComponent = true`.
+ *
  * ```html
  * <vaadin-breadcrumb>
  *   <vaadin-breadcrumb-item href="/">Home</vaadin-breadcrumb-item>

packages/breadcrumb/src/vaadin-breadcrumb.js

@@ -15,6 +15,8 @@ import { breadcrumbStyles } from './styles/vaadin-breadcrumb-styles.js';
 /**
  * `<vaadin-breadcrumb>` is a Web Component for displaying hierarchical navigation.
  *
+ * ⚠️ **This component is experimental** and the API may change. In order to use it, enable the feature flag by setting `window.Vaadin.featureFlags.breadcrumbComponent = true`.
+ *
  * ```html
  * <vaadin-breadcrumb>
  *   <vaadin-breadcrumb-item href="/">Home</vaadin-breadcrumb-item>
@@ -43,6 +45,10 @@ class Breadcrumb extends ElementMixin(ThemableMixin(PolylitMixin(LumoInjectionMi
     return 'vaadin-breadcrumb';
   }
 
+  static get experimental() {
+    return true;
+  }
+
   static get styles() {
     return breadcrumbStyles;
   }

packages/breadcrumb/test/breadcrumb.test.js

@@ -1,5 +1,11 @@
 import { expect } from '@vaadin/chai-plugins';
 import { fixtureSync, nextFrame } from '@vaadin/testing-helpers';
+
+// Enable experimental breadcrumb component
+window.Vaadin ||= {};
+window.Vaadin.featureFlags ||= {};
+window.Vaadin.featureFlags.breadcrumbComponent = true;
+
 import '../vaadin-breadcrumb.js';
 import '../vaadin-breadcrumb-item.js';