add vitest alongside jest (phase 0)

vitest/vitestSetup.ts

@@ -0,0 +1,16 @@
+import {vi} from 'vitest'
+
+/*
+ * The only place `react-native` enters the test module graph is a single
+ * transitive import in src/lib/media/manip.ts (`import {Image} from
+ * 'react-native'`). react-native's entry is Flow-typed, which esbuild cannot
+ * parse, so we stub it. manip.ts only touches RNImage.getSize; Platform is
+ * included for any other incidental use.
+ */
+vi.mock('react-native', () => ({
+  Image: {getSize: vi.fn()},
+  Platform: {
+    OS: 'ios',
+    select: (o: Record<string, unknown>) => o.ios ?? o.default,
+  },
+}))