refactor(notifications): extract routing predicates and fix pre-overlay

- Extract shouldShowOsd/Overlay/Desktop into notification-routing.ts (was duplicated in 3 files)
- Add resolveOverlayReadinessNotificationType: preserves system channel when overlay not ready (both→osd-system, system→system, overlay→osd)
- Route overlay loading status through showConfiguredStatusNotification instead of raw OSD

src/main/runtime/notification-routing.test.ts

@@ -0,0 +1,29 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  resolveOverlayReadinessNotificationType,
+  shouldShowDesktop,
+  shouldShowOverlay,
+  shouldShowOsd,
+} from './notification-routing';
+
+test('notification routing preserves system notification while overlay is not ready', () => {
+  assert.equal(resolveOverlayReadinessNotificationType('system', false), 'system');
+});
+
+test('notification routing preserves both as osd plus system while overlay is not ready', () => {
+  assert.equal(resolveOverlayReadinessNotificationType('both', false), 'osd-system');
+});
+
+test('notification routing falls back overlay-only notification to osd while overlay is not ready', () => {
+  assert.equal(resolveOverlayReadinessNotificationType('overlay', false), 'osd');
+});
+
+test('notification routing predicates classify delivery channels', () => {
+  assert.equal(shouldShowOverlay('both'), true);
+  assert.equal(shouldShowOverlay('system'), false);
+  assert.equal(shouldShowOsd('osd-system'), true);
+  assert.equal(shouldShowOsd('both'), false);
+  assert.equal(shouldShowDesktop('osd-system'), true);
+  assert.equal(shouldShowDesktop('overlay'), false);
+});