Summary

Implement the runtime substrate for ZEP-0002 Structured Async: fibers plus the work-stealing thread pool they run on.

Scope

• Fiber context switching abstraction for Linux, macOS, and Windows
• Runtime initialization/deinitialization API for thread-pool setup
• One worker thread per logical CPU by default, configurable via runtime config / ZAG_THREADS
• Local run queues plus stealing strategy for idle workers
• Fiber lifecycle, stack allocation/growth strategy, and failure handling
• Minimal internal tracing/metrics hooks needed for validation

Acceptance criteria

• Runtime boots and shuts down cleanly on supported host platforms
• Fibers can be scheduled, parked, resumed, and completed without leaking memory
• Idle workers can steal runnable work from other workers
• Default thread count and explicit override both work
• Design constraints in ZEP-0002 Implementation Notes are reflected in code and docs

References

• ZEP-0002 Phase 1 and Done Criteria
• docs/zep/ZEP-0002-async-structure.md
• docs/adr/ADR-0002-beam-concurrency-gap-analysis.md