chore: scaffold @insignia/iios-messaging-ui with jsdom test setup

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/iios-messaging-ui/tsup.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'tsup';
+
+export default defineConfig({
+  // `src/adapters/mock.ts` and `src/conformance.ts` are added as separate entries
+  // in later tasks (they don't exist yet). `conformance` in particular is its own
+  // entry, never reachable from `index`, because it imports vitest, and bundling
+  // that into the main barrel would drag a test runner into every consumer's
+  // production build.
+  entry: ['src/index.ts'],
+  format: ['esm'],
+  dts: true,
+  clean: true,
+  external: ['react', 'react-dom', 'vitest'],
+});