Async runtime (Phase 10) for chain/clock.py and networking

[pirapira]

Summary

Phase 10 of the development plan: implement async/await support for leanSpec's networking and node code. Currently, chain/clock.py imports asyncio at module level, blocking its import.

Current behavior

• import asyncio raises ModuleNotFoundError
• chain/clock.py cannot be imported (defines SlotClock with async methods)
• All networking, sync, validator service, and API modules use async/await

Scope

Minimal async subset needed:

• async def, await, async for, async with
• asyncio.create_task, asyncio.gather, asyncio.sleep
• asyncio.Queue, asyncio.Event
• Basic event loop
• Task cancellation

Impact

Blocks ~23 leanSpec files (~9,100 lines):

• chain/clock.py, chain/service.py
• subspecs/networking/ (discovery, gossipsub, reqresp)
• subspecs/sync/ (checkpoint, head, backfill sync)
• subspecs/validator/ (validator service)
• subspecs/api/ (REST endpoints)
• subspecs/node/ (full node orchestration)

Reference

• PLAN.md Phase 10
• references/leanSpec/src/lean_spec/subspecs/chain/clock.py