feat: salvage Angular developer skill

skills/angular-developer/references/testing-fundamentals.md

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+# Testing Fundamentals
+
+This guide covers the fundamental principles and practices for writing Angular unit and component tests. Use the runner already configured in the project.
+
+## Core Philosophy: Async-First
+
+Modern Angular applications often schedule state changes asynchronously, especially when using signals or zoneless change detection. Tests should account for this.
+
+Prefer the "Act, Wait, Assert" pattern:
+
+1. **Act:** Update state or perform an action (e.g., set a component input, click a button).
+2. **Wait:** Use `await fixture.whenStable()` to allow the framework to process the scheduled update and render the changes.
+3. **Assert:** Verify the outcome.
+
+### Basic Test Structure Example
+
+```ts
+import {ComponentFixture, TestBed} from '@angular/core/testing';
+import {MyComponent} from './my.component';
+
+describe('MyComponent', () => {
+  let component: MyComponent;
+  let fixture: ComponentFixture<MyComponent>;
+  let h1: HTMLElement;
+
+  beforeEach(async () => {
+    // 1. Configure the test module
+    await TestBed.configureTestingModule({
+      imports: [MyComponent],
+    }).compileComponents();
+
+    // 2. Create the component fixture
+    fixture = TestBed.createComponent(MyComponent);
+    component = fixture.componentInstance;
+    h1 = fixture.nativeElement.querySelector('h1');
+  });
+
+  it('should display the default title', async () => {
+    // ACT: (Implicit) Component is created with default state.
+    // WAIT for initial data binding.
+    await fixture.whenStable();
+    // ASSERT the initial state.
+    expect(h1.textContent).toContain('Default Title');
+  });
+
+  it('should display a different title after a change', async () => {
+    // ACT: Change the component's title property.
+    component.title.set('New Test Title');
+
+    // WAIT for the asynchronous update to complete.
+    await fixture.whenStable();
+
+    // ASSERT the DOM has been updated.
+    expect(h1.textContent).toContain('New Test Title');
+  });
+});
+```
+
+## TestBed and ComponentFixture
+
+- **`TestBed`**: The primary utility for creating a test-specific Angular module. Use `TestBed.configureTestingModule({...})` in your `beforeEach` to declare components, provide services, and set up imports needed for your test.
+- **`ComponentFixture`**: A handle on the created component instance and its environment.
+  - `fixture.componentInstance`: Access the component's class instance.
+  - `fixture.nativeElement`: Access the component's root DOM element.
+  - `fixture.debugElement`: An Angular-specific wrapper around the `nativeElement` that provides safer, platform-agnostic ways to query the DOM (e.g., `debugElement.query(By.css('p'))`).