tokio-based runtime helpers for geph5.
This crate replaces the smol/smolscale runtime surface (spawning, blocking,
timeouts, the immortal/respawn pattern, and the task reaper) while preserving
the property the codebase relies on for structured concurrency: smol's
Task<T> cancels the task when the handle is dropped. tokio's
JoinHandle detaches on drop instead, so [Task] reintroduces drop-cancel.