Avoid busy-spin waits in zombie cleanup lock

[shaun0927]

Summary

• Replaces the zombie-cleanup registry lock busy-spin wait with a dependency-free synchronous sleep helper.
• Preserves existing lock retry interval, stale timeout, synchronous registry APIs, and failure behavior.
• Adds targeted unit coverage for the wait helper and keeps cross-session zombie cleanup tests green.

Closes #754.

Direction/scope review

• Aligned with OpenSafari simulator lifecycle reliability and CI stability.
• Low risk: no behavior or timeout changes, only removes CPU-burning spin wait.
• No dependencies or registry redesign.

Verification

• npm test -- --runTestsByPath tests/unit/zombie-cleanup-lock.test.ts tests/unit/zombie-cleanup-cross-session.test.ts
• npm run lint -- --quiet src/reliability/zombie-cleanup.ts tests/unit/zombie-cleanup-lock.test.ts
• npm run build

Live validation after merge

• Contend the registry lock from two OpenSafari processes and confirm the waiter does not burn a full CPU core.
• Confirm active registered simulators remain protected from cleanup.

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