perf(client): reduce thread-list update work - #9716

💭 The diff only touches threadShell.ts within scope, so I need to check that file against Effect service conventions.

1. browse_code — packages/client-runtime/src/state/threadShell.ts

💭 This is atom/reactivity code rather than an Effect service, so the service conventions around imports, errors, and runtime boundaries don't really apply here. The import from the reactivity subpath is pre-existing, so I'm focusing on the actual changes: the WeakMap memoization and the previous project IDs comparison logic. I'm not seeing any real violations of the scoped conventions — the closure use isn't hiding a service dependency. I'll mark this as clear with a short success summary.