Playground cache leases cannot traverse directory descriptors on macOS

[chubes4]

Problem

Merged #1930 uses /dev/fd/<directory-fd> as an anchored directory path on Darwin/FreeBSD. Node on macOS can stat that descriptor but cannot enumerate it or open children through it. Any wp: latest runtime that acquires or inspects a custom archive lease now fails locally.

Reproduction

On macOS with current main (6bbfc836):

• readdir('/dev/fd/<directory-fd>') fails ENOTDIR.
• open('/dev/fd/<directory-fd>/<lease>.json') fails ENOENT.
• npm run test:playground-custom-archive-cache fails in createFileLease() at playground-wordpress-archive-cache.ts:381.
• Switching only directory enumeration back to the validated pathname gets farther, then child creation fails through the descriptor as above.

Acceptance criteria

• Darwin local Playground runtimes can acquire, inspect, renew, and release archive leases.
• The replacement/symlink race guarantees from Harden Playground cache retention safety #1930 remain explicit and covered; do not silently downgrade to unanchored pathname access.
• npm run test:playground-custom-archive-cache and a wp: latest runtime integration pass on macOS.