feat(storybook): wire @storybook/test-runner for story smoke tests

Every story is now executed as a smoke test (mount + no console errors)
via @storybook/test-runner. New script: pnpm test:stories runs
build-storybook then test-storybook against the static build.

Spec: §6.8

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

turbo.json

@@ -51,6 +51,13 @@
     },
     "typecheck": {
       "dependsOn": ["^typecheck"]
+    },
+    "build-storybook": {
+      "outputs": ["storybook-static/**"]
+    },
+    "test:stories": {
+      "dependsOn": ["build-storybook"],
+      "cache": false
     }
   }
 }