feat(shared): add createSingleton lazy-singleton accessor helper

Standardizes the getX() / __resetX() pattern hand-rolled identically across
every store: lazy construction on first get(), memoized thereafter, and a
reset() that runs an optional teardown (e.g. destroy()) and clears so the
next get() rebuilds. Lazy by construction, so owning modules stay inert at
import. Covered by unit tests (laziness, memoization, rebuild-after-reset,
teardown-once-with-live-instance, reset-before-get no-op, falsy-value
caching).

Not yet adopted by the stores — that migration is a separate step.

src/shared/lib/helpers/createSingleton/createSingleton.test.ts

@@ -0,0 +1,86 @@
+import {
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { createSingleton } from './createSingleton';
+
+describe('createSingleton', () => {
+    it('does not call the factory until the first get (lazy)', () => {
+        const factory = vi.fn(() => ({ id: 1 }));
+        createSingleton(factory);
+        expect(factory).not.toHaveBeenCalled();
+    });
+
+    it('constructs on first get and memoizes the instance', () => {
+        const factory = vi.fn(() => ({ id: 1 }));
+        const singleton = createSingleton(factory);
+
+        const a = singleton.get();
+        const b = singleton.get();
+
+        expect(factory).toHaveBeenCalledTimes(1);
+        expect(a).toBe(b);
+    });
+
+    it('rebuilds a fresh instance after reset', () => {
+        let count = 0;
+        const singleton = createSingleton(() => ({ id: ++count }));
+
+        const first = singleton.get();
+        singleton.reset();
+        const second = singleton.get();
+
+        expect(first).not.toBe(second);
+        expect(second.id).toBe(2);
+    });
+
+    it('runs teardown once, with the live instance, on reset', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        const instance = singleton.get();
+        singleton.reset();
+
+        expect(teardown).toHaveBeenCalledTimes(1);
+        expect(teardown).toHaveBeenCalledWith(instance);
+    });
+
+    it('treats reset before any get as a no-op (no teardown, no throw)', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        expect(() => singleton.reset()).not.toThrow();
+        expect(teardown).not.toHaveBeenCalled();
+    });
+
+    it('does not run teardown again on a second consecutive reset', () => {
+        const teardown = vi.fn();
+        const singleton = createSingleton(() => ({ id: 1 }), teardown);
+
+        singleton.get();
+        singleton.reset();
+        singleton.reset();
+
+        expect(teardown).toHaveBeenCalledTimes(1);
+    });
+
+    it('works without a teardown', () => {
+        const singleton = createSingleton(() => ({ id: 1 }));
+
+        singleton.get();
+        expect(() => singleton.reset()).not.toThrow();
+        expect(singleton.get().id).toBe(1);
+    });
+
+    it('caches a falsy instance value without re-running the factory', () => {
+        const factory = vi.fn(() => undefined);
+        const singleton = createSingleton<undefined>(factory);
+
+        singleton.get();
+        singleton.get();
+
+        expect(factory).toHaveBeenCalledTimes(1);
+    });
+});