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/turbo.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://turborepo.dev/schema.json",
+  "extends": ["//"],
+  "tags": ["core"]
+}