Evaluate shared worker runtime

[taskylizard]

A shared Tokio runtime with dedicated isolates could reduce memory usage compared to per-worker runtimes.

Ref: apps/runtime/src/runtime/worker.rs (spawn_worker)

[taskylizard]

Closing as not planned. Shared-worker runtime migration is blocked by deno_unsync requiring current_thread runtimes for isolates. Kept only bounded worker queues + backpressure; dropped shared scheduler/runtime migration path.