Add idle timeout for WebWorkerPool

[paberr]

Summary

• Adds idle_timeout_ms option to WorkerPoolOptions — workers with no pending tasks are terminated after the configured duration and transparently recreated on demand
• Uses per-slot RefCell<WorkerSlot> so the idle checker and task execution can borrow independent slots without conflict
• Adds last_active() tracking to WebWorker, num_active_workers() to WebWorkerPool, and integration tests for idle cleanup

Test plan

• cargo +nightly fmt --all -- --check passes
• cargo +nightly clippy --all-features passes (no warnings)
• cargo test --doc --all-features passes
• cd test && npm ci && npm test (4/4 integration tests pass)

[Copilot]

Pull request overview

Adds an idle-timeout feature to WebWorkerPool so that inactive web workers can be terminated after a configurable duration and recreated on-demand, with supporting scheduler updates and integration coverage in the test crate.

Changes:

• Add idle_timeout_ms to WorkerPoolOptions and an interval-based idle reaper that transitions workers to an Empty slot state.
• Refactor the pool from Vec<WebWorker> to per-slot RefCell<WorkerSlot> and update scheduling to operate on per-slot load snapshots.
• Add WebWorker::last_active() tracking and an integration test validating idle termination and transparent recreation.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/pool/mod.rs	Introduces idle-timeout configuration, WorkerSlot state machine, slot-based worker management, recreation logic, and active-worker metrics.
src/pool/scheduler.rs	Updates scheduler API to choose from Option<load> slots and return None when no active workers exist.
src/webworker/worker.rs	Adds last_active timestamp tracking updated on task completion for idle-timeout decisions.
test/src/raw.rs	Adds a JS sleep_ms helper and a new integration test for idle cleanup + recreation.
test/src/lib.rs	Wires the new idle-timeout test into the test runner.
test/Cargo.toml	Adds js-sys dependency and enables web-sys Window feature for timer usage.
Cargo.lock	Updates lockfile for the new js-sys dependency in the test crate.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.