Summary

Add the runtime primitives behind Preview 3 draft waitables, waitable sets, readiness, backpressure, and cancellation so async package APIs have something concrete to run on.

Parent tracking issue: #70
Suggested implementation order: 4 / 14

References

• https://github.com/WebAssembly/WASI/blob/main/docs/Preview2.md
• https://github.com/WebAssembly/component-model/blob/2e4800866d529aedc1b7730bc9452a4f85b97d8f/design/mvp/Concurrency.md

Acceptance criteria

• The runtime can represent waitables and readiness transitions needed by Preview 3 draft async flows.
• Backpressure and cancellation paths behave predictably instead of dropping work or hanging tasks indefinitely.
• Tests cover blocking, wakeups, cancellation, and closure behavior across at least one representative native and web path.