feat: minimal signals + RemoteData template with a users feature

Extracted from atomic-design-poc: RemoteData<E,T> (bare union + fromResource,
no combinators) + a trimmed <app-async> switch, no Elm-style store — state is
resource() plus one plain signal. Minimal DDD layering per context
(domain/infrastructure/application/ui) combined with atomic design inside
ui/ (atoms/molecules/organisms/templates/pages), mirroring the POC's
conventions at template scale.

One worked feature (users/): a list with a click-through to a detail view
(its own independent resource() fetch) and a Back action. Tests are
BDD-style (describe/it, one assertion per it) and black-box (assert
rendered DOM/emitted events only) - 100% branch/line coverage. README.md
documents what's deliberately omitted vs. the POC and the concrete growth
path back to it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/users/domain/user.spec.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'vitest';
+import { isEmptyUserList } from './user';
+
+describe('isEmptyUserList', () => {
+  it('is empty for an empty list', () => {
+    expect(isEmptyUserList([])).toBe(true);
+  });
+
+  it('is not empty when a user is present', () => {
+    expect(isEmptyUserList([{ id: 1, name: 'Ada' }])).toBe(false);
+  });
+
+  it('is empty when the list is undefined', () => {
+    expect(isEmptyUserList(undefined)).toBe(true);
+  });
+});

src/app/users/domain/user.ts

@@ -0,0 +1,15 @@
+export interface User {
+  id: number;
+  name: string;
+}
+
+export interface UserDetail extends User {
+  email: string;
+}
+
+/**
+ * Accepts `undefined` because a resource's value is `T | undefined` until it resolves
+ * (no `defaultValue` is set) — `fromResource` only calls this once `hasValue()` is true,
+ * but the type can't express that across the generic boundary.
+ */
+export const isEmptyUserList = (users: User[] | undefined): boolean => !users || users.length === 0;