Add worker hooks for deterministic tests

[leynos]

Summary

• expose WorkerHooks to signal queue progress
• allow graceful worker shutdown via watch channel
• rewrite worker tests to wait for hooks instead of polling

Testing

• make fmt
• make lint
• make test
• make test-cov (fails: cargo-llvm-cov not found)
• make markdownlint
• make nixie

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https://chatgpt.com/codex/tasks/task_e_68a200c0fcbc8322aa6fc670ade46141

Summary by Sourcery

Add WorkerHooks for observable worker lifecycle events and integrate graceful shutdown support, refactor run_worker and daemon wiring accordingly, and update tests to use hooks for deterministic synchronization.

New Features:

• Introduce WorkerHooks struct with optional Notify hooks (enqueued, idle, drained) for deterministic test synchronization
• Enable graceful worker shutdown via a watch channel

Enhancements:

• Refactor run_worker and run_listener to accept a shutdown channel and WorkerHooks, and integrate select-based shutdown logic
• Update daemon main and design documentation to wire up the shutdown channel and default WorkerHooks

Documentation:

• Update design docs to reflect the new run_worker signature and shutdown/hooks usage

Tests:

• Rewrite worker tests to await WorkerHooks notifications using Notify and timeout instead of polling utilities
• Remove polling-based test utilities and related teardown logic