Fix Playground cache leases on macOS

[chubes4]

Summary

• retain descriptor-relative lease access on Linux
• introduce an explicit generation-checked pathname strategy for Darwin and FreeBSD, where Node cannot traverse /dev/fd/<directory-fd>
• validate the parent directory generation before and after enumeration, child inspection, open, and unlink operations
• fail closed when the sidecar path no longer matches the opened directory
• cover parent-directory replacement during Darwin/FreeBSD lease release

Root cause

Node on macOS can stat /dev/fd/<directory-fd> but readdir() fails with ENOTDIR and child opens fail with ENOENT. #1930 treated Darwin descriptor paths as Linux-compatible, breaking all custom WordPress archive lease acquisition.

Verification

• npm run build
• npm run test:playground-custom-archive-cache
• git diff --check

Fixes #1934