fix: serialize readonly mount cache retention

[chubes4]

Summary

• use one cache-root lock for readonly snapshot creation, retention, and cloning
• prevent retention for one fingerprint from evicting another parallel mount's active cache entry
• cover twelve unique parallel mounts, exceeding the eight-entry retention limit

Verification

• npm exec -- tsx tests/playground-readonly-mounts.test.ts
• npm run build
• full released Gardner recipe completed successfully: run run_8cc6c2afdd3b43feae01ccd4b5683044, runtime runtime-mt8rh00m-p0b89z

Fixes #2382