feat(sdk): P2.6 @insignia/iios-message-web (React hooks) + allowlist boundary

MessageProvider/useThread/useMessages over the kernel-client socket (live append,
typing, dedupe). Reconnect test (re-opens active thread). Rewrote import-boundary
as an allowlist so SDK<->service sibling imports are forbidden both ways. 28 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-message-web/tsup.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'tsup';
+
+export default defineConfig({
+  entry: ['src/index.ts'],
+  format: ['esm'],
+  dts: true,
+  clean: true,
+  external: ['react', 'react-dom', '@insignia/iios-kernel-client'],
+});