test(engine): guard offline-loop safety and harden purity test

- save.test.ts: add regression test proving loop actions do NOT start
  or yield during offline catch-up (applyOfflineProgress replays
  tickGame directly; maybeStartLoopAction is never called, so an
  enabled loop with no active action stays idle and yields 0).

- purity.test.ts: extend FORBIDDEN list to catch environment APIs
  (Date.now(), localStorage., indexedDB., idb-keyval import, document.,
  window., requestAnimationFrame()) in addition to the existing
  React/react-dom/zustand import guards. Patterns are scoped to call
  sites and member-access forms so prose comments (e.g. save.ts's
  "Date scheduling" doc comment) do not false-positive. lz-string is
  intentionally omitted — it is a pure compression library.

- NavRail.tsx: add sr-only "New story" span alongside the aria-hidden
  pulse dot so screen readers can perceive the unread-story badge.

src/engine/__tests__/purity.test.ts

@@ -3,7 +3,40 @@ import { join } from 'node:path';
 import { describe, expect, it } from 'vitest';
 
 const ENGINE_DIR = join(import.meta.dirname, '..');
-const FORBIDDEN = [/from\s+['"]react/, /from\s+['"]react-dom/, /from\s+['"]zustand/];
+
+/**
+ * Patterns that must NOT appear in engine source files.
+ *
+ * React / state-manager imports — match the import statement so plain
+ * string occurrences in comments are not flagged.
+ *
+ * Environment / browser / wall-clock APIs — matched as usage tokens
+ * (property-access or call-site forms) to avoid false-positives on
+ * prose comments that name these APIs without using them.  In
+ * particular:
+ *   - `Date\.now\(` catches the call site; a comment saying "Date
+ *     scheduling" does not contain "Date.now(" so it passes.
+ *   - `localStorage\.` / `indexedDB\.` catch member-access, not the
+ *     bare words that appear in save.ts's module-doc comment.
+ *   - `from\s+['"]idb-keyval` catches the package import.
+ *   - `\bdocument\.` / `\bwindow\.` catch DOM member-access.
+ *   - `requestAnimationFrame\(` catches the call site.
+ *
+ * lz-string is a pure compression library used by save.ts — it is
+ * intentionally NOT in this list.
+ */
+const FORBIDDEN: { pattern: RegExp; label: string }[] = [
+  { pattern: /from\s+['"]react['"]/, label: 'react import' },
+  { pattern: /from\s+['"]react-dom['"]/, label: 'react-dom import' },
+  { pattern: /from\s+['"]zustand['"]/, label: 'zustand import' },
+  { pattern: /Date\.now\(/, label: 'Date.now() call (wall-clock)' },
+  { pattern: /localStorage\./, label: 'localStorage access (storage API)' },
+  { pattern: /indexedDB\./, label: 'indexedDB access (storage API)' },
+  { pattern: /from\s+['"]idb-keyval['"]/, label: 'idb-keyval import (storage API)' },
+  { pattern: /\bdocument\./, label: 'document access (DOM API)' },
+  { pattern: /\bwindow\./, label: 'window access (browser global)' },
+  { pattern: /requestAnimationFrame\(/, label: 'requestAnimationFrame call (scheduling API)' },
+];
 
 async function engineSourceFiles(): Promise<string[]> {
   const entries = await readdir(ENGINE_DIR, { withFileTypes: true });
@@ -18,8 +51,8 @@ describe('engine purity', () => {
     expect(files.length).toBeGreaterThan(0);
     for (const file of files) {
       const source = await readFile(file, 'utf8');
-      for (const pattern of FORBIDDEN) {
-        expect(source, `${file} must stay free of ${pattern}`).not.toMatch(pattern);
+      for (const { pattern, label } of FORBIDDEN) {
+        expect(source, `${file} must not use ${label}`).not.toMatch(pattern);
       }
     }
   });

src/engine/__tests__/save.test.ts

@@ -135,6 +135,35 @@ describe('invalid / tampered saves', () => {
   });
 });
 
+describe('applyOfflineProgress() — loop invariant', () => {
+  it('does NOT start a loop action during offline catch-up even when it is enabled', () => {
+    // Hardest invariant: maybeStartLoopAction is called by the runtime AFTER each live
+    // tick, never from tickGame itself. Offline catch-up replays tickGame directly, so
+    // loop actions must never start (and therefore never yield) during catch-up.
+    const content = buildContent({
+      resources: [{ id: 'wood', name: 'Wood', startAmount: 0 }],
+      actions: [
+        {
+          id: 'chop',
+          name: 'Chop Wood',
+          kind: 'loop',
+          group: { id: 'test', label: 'Test' },
+          durationMs: 1000,
+          yields: [{ resourceId: 'wood', amount: 1 }],
+        },
+      ],
+    });
+    const state = createGameState(content);
+    // Enable the loop — player has toggled it on — but do NOT make it active.
+    state.enabledLoopActionIds.chop = true;
+    // Simulate coming back online after 10 seconds (10 full loop durations).
+    applyOfflineProgress(state, content, 0, 10_000);
+    // The loop must NOT have started or yielded during offline catch-up.
+    expect(state.activeActionId).toBeNull();
+    expect(state.resources.wood).toBe(0);
+  });
+});
+
 describe('applyOfflineProgress()', () => {
   it('credits whole ticks of elapsed time to the active action', () => {
     const content = testContent();

src/ui/NavRail.tsx

@@ -122,10 +122,13 @@ export function NavRail() {
               {item.icon}
               <span className="hidden md:inline">{item.label}</span>
               {item.id === 'story' && storyHasUnread ? (
+                <>
                   <span
                     className="absolute top-1.5 right-1.5 md:top-1/2 md:right-3 h-2 w-2 md:-translate-y-1/2 animate-pulse rounded-full bg-amber-500 shadow-[0_0_8px_rgba(245,158,11,0.6)]"
                     aria-hidden="true"
                   />
+                  <span className="sr-only">New story</span>
+                </>
               ) : null}
             </button>
           );