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/shared/ui/atoms/spinner.component.ts

@@ -0,0 +1,22 @@
+import { Component } from '@angular/core';
+
+@Component({
+  selector: 'app-spinner',
+  template: `<div class="spinner" role="status" aria-label="Loading"></div>`,
+  styles: `
+    .spinner {
+      width: 1.5rem;
+      height: 1.5rem;
+      border: 2px solid #ddd;
+      border-top-color: #333;
+      border-radius: 50%;
+      animation: spin 0.6s linear infinite;
+    }
+    @keyframes spin {
+      to {
+        transform: rotate(360deg);
+      }
+    }
+  `,
+})
+export class SpinnerComponent {}