Fix concurrent Playground cache lease handoffs

[chubes4]

Summary

• retry archive-lock acquisition when a waiting reader observes a completed owner removing its sidecar
• verify the opened directory generation before every pathname-based lock-directory removal, including Linux cleanup
• add a barrier-synchronized two-process test that proves one materializer, successful waiting handoffs, and stale-owner recovery

Fixes #2247.

Verification

• npm run test:playground-custom-archive-cache
• npm run build
• two independent 500-iteration process loops against a fresh 7.0.3.zip.lock: both exit 0 after the fix; current main reproduced Playground cache lease sidecar changed while accessing in one process

AI disclosure

Implemented with openai/gpt-5.6-sol via OpenCode. Chris Huber reviewed and remains responsible for the change.