test(ui-sidebar): pin the shell chrome with local DOM snapshots

First consumer of the runtime's single-slot mounting: the real apply mounts
on its own fiber, renderSlot('sidebar', ...) captures exactly the sidebar
slot's output, and update() re-renders the collapsed rail in place. The
.snap files carry semantic class names and svg fingerprints only.

packages/client/ui-sidebar/package.json

@@ -47,6 +47,7 @@
   },
   "devDependencies": {
     "@deepseek-ai/dsh-client-runtime": "workspace:^",
+    "@deepseek-ai/dsh-client-test-runtime": "workspace:^",
     "@deepseek-ai/dsh-client-ui-layout": "workspace:^",
     "@deepseek-ai/dsh-client-ui-primitives": "workspace:^",
     "@deepseek-ai/dsh-client-ui-slots": "workspace:^",