Reuse readonly Playground mount snapshots

[chubes4]

Summary

• reuse source-generation-verified readonly directory snapshots across unchanged runs
• serialize concurrent preparation with renewable leases
• clone prepared snapshots per runtime using copy-on-write when supported
• preserve symlink escape/cycle safety and nested overlay behavior
• report preparation mode, reuse, bytes, files, and elapsed time
• bound retained snapshots by age and count

Closes #2222.

Verification

• npm run build
• npm run test:playground-readonly-mounts
• npm run test:playground-readonly-mounts-integration
• git diff --check

Filesystems without reflink/copy-on-write support safely fall back to a physical per-runtime copy.

AI assistance

• Model: OpenAI gpt-5.6-sol
• Tool: OpenCode
• Use: A general coding agent implemented and tested the reusable staging contract under Chris Huber direction. Chris remains responsible for every line.