feat(core-realtime): scaffold realtime optional core

Generator-emitted scaffold (pnpm turbo gen core-package realtime) plus
the story-00-precedent coverage repairs (coverage provider devDep,
symbols.ts exclude + tested allowlist mirror) and three minimal tests
covering generator-emitted realtime code the template suite misses.
Squash of 31d85e0 + review-fix cf11b38.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-eslint/rules/no-direct-socket-io.test.js

@@ -0,0 +1,48 @@
+// packages/core-eslint/rules/no-direct-socket-io.test.js
+import { RuleTester } from "eslint";
+import rule from "./no-direct-socket-io.js";
+
+const tester = new RuleTester({
+  languageOptions: { ecmaVersion: 2022, sourceType: "module" },
+});
+
+tester.run("no-direct-socket-io", rule, {
+  valid: [
+    // Allowed inside core-realtime
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/packages/core-realtime/src/socket-io-realtime-server.ts",
+    },
+    // Allowed in app servers
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/apps/web-next/server.ts",
+    },
+    // Allowed in app integration tests (e.g. realtime-ping e2e)
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/apps/web-next/src/__tests__/realtime-ping.test.ts",
+    },
+    {
+      code: 'import { io } from "socket.io-client";',
+      filename: "/repo/apps/web-next/src/__tests__/realtime-ping.test.ts",
+    },
+    // Allowed elsewhere when not importing socket.io
+    {
+      code: 'import { foo } from "bar";',
+      filename: "/repo/packages/blog/src/foo.ts",
+    },
+  ],
+  invalid: [
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/packages/blog/src/foo.ts",
+      errors: [{ messageId: "noDirectSocketIO" }],
+    },
+    {
+      code: 'import { io } from "socket.io-client";',
+      filename: "/repo/packages/blog/src/ui/Component.tsx",
+      errors: [{ messageId: "noDirectSocketIOClient" }],
+    },
+  ],
+});