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-realtime/vitest.config.ts

@@ -0,0 +1,18 @@
+import path from "node:path";
+import { mergeConfig } from "vitest/config";
+import { nodeVitestConfig } from "@repo/core-typescript/vitest.base.node";
+
+export default mergeConfig(nodeVitestConfig, {
+  test: {
+    coverage: {
+      exclude: [
+        // DI symbol constants — boilerplate, covered implicitly by bind-* tests
+        // (mirrors core-shared/vitest.config.ts)
+        "src/**/symbols.ts",
+      ],
+    },
+  },
+  resolve: {
+    alias: { "@": path.resolve(__dirname, "./src") },
+  },
+});