Controlled Svelte cleanup diagnostic

Hx Works ran a self-initiated lifecycle check with Svelte 5.23.2: two self-written components, real mount/unmount, and a ControlledEventSource mock.

Both cases waited for the same Promise step and the setup-ready signal before unmount. In two recorded browser runs: • Async onMount callback: returned-cleanup invocations 0; close() calls 0. • Synchronous-return control: returned-cleanup invocations 1; close() calls 1. • Both: mounted DOM removed; onDestroy hook called once.

Observed event order: setup ready → unmount requested → onDestroy. Only the synchronous-return control then invoked cleanup and close().

Unclosed counts refer only to mock objects. The ControlledEventSource mock opened no network connection. Real HTTP/SSE, retained-memory measurement, unmount during pending setup, and cross-browser behavior were not tested. The synchronous control is not a complete production patch.

Self-initiated research demonstration.