feat(messaging-ui): Inbox domain — unified work items + mail (contract + UI + mock)

Second domain in the SDK, mirroring messaging's adapter/provider/hooks/components:
- InboxAdapter (listInbox unified feed + transition + mailHistory/mailReply +
  optional directory/composeInternal/composeExternal), InboxProvider/useInboxAdapter
- hooks: useInbox (filter + transition), useMailThread (history + reply), useCompose
- <Inbox> (filter tabs, unified list, detail pane), <MailReader> (HTML in a
  sandboxed iframe + reply), compose modal (in-app / email); themeable styles
- MockInboxAdapter (seeded mentions/needs-reply/alert + folded mail threads +
  directory) shipped from ./adapters/mock-inbox
- 6 inbox tests (mock unify/transition/reply + render open/reply/compose); 64 green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -6,10 +6,10 @@ export default defineConfig({
   // 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', 'src/adapters/mock.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts', 'src/styles.css'],
+  entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/mock-inbox.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts', 'src/styles.css'],
   format: ['esm'],
   // Types only for the TS entries — styles.css has no .d.ts (and tsc chokes on a .css root file).
-  dts: { entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts'] },
+  dts: { entry: ['src/index.ts', 'src/adapters/mock.ts', 'src/adapters/mock-inbox.ts', 'src/adapters/kernel-client.ts', 'src/conformance.ts'] },
   clean: true,
   external: ['react', 'react-dom', 'vitest', '@insignia/iios-kernel-client'],
 });