feat(kernel-browser-runtime): add UI vat infrastructure with slot-based orchestration

Add UIOrchestrator for managing visible UI vat iframes in named slots,
enabling multiple caplet UIs to coexist in a structured layout.

- Add UIOrchestrator class with slot-based iframe management
- Add makeUIVatWorker factory for VatWorker interface
- Add <div id="root"> mounting point to vat/iframe.html
- Export SlotName type (currently 'main', extensible)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sirtimid

.gitignore

@@ -92,3 +92,4 @@ test-results
 
 # Claude
 .claude/settings.local.json
+.playwright-mcp/

packages/kernel-browser-runtime/src/index.test.ts

@@ -7,6 +7,7 @@ describe('index', () => {
     expect(Object.keys(indexModule).sort()).toStrictEqual([
       'PlatformServicesClient',
       'PlatformServicesServer',
+      'UIOrchestrator',
       'connectToKernel',
       'createRelayQueryString',
       'getCapTPMessage',
@@ -17,6 +18,7 @@ describe('index', () => {
       'makeBackgroundCapTP',
       'makeCapTPNotification',
       'makeIframeVatWorker',
+      'makeUIVatWorker',
       'parseRelayQueryString',
       'receiveInternalConnections',
       'rpcHandlers',

packages/kernel-browser-runtime/src/index.ts

@@ -21,3 +21,4 @@ export {
   type BackgroundCapTPOptions,
   type CapTPMessage,
 } from './background-captp.ts';
+export * from './ui/index.ts';