Retry Playground archive lock generation handoffs

## Problem

The authoritative smoke aggregate for #2463 exposed a cross-process handoff that the lock protocol detected correctly but classified incorrectly. `tryAcquireCacheLock()` can create the shared lock directory and then be descheduled before opening it. After the bounded empty-directory grace expires, another process may legitimately reclaim and replace that uninitialized generation. The original creator then observes a generation change between `lstat()` and `open()`.

Observed failure: [smoke aggregate run 33561166359](https://github.com/Automattic/wp-codebox/actions/runs/33561166359/job/100033725647)

```text
LeaseSidecarUnsafeError: Playground cache lease sidecar changed while opening
```

The protocol already retried generation changes detected while accessing an opened sidecar, but identified that outcome by matching one error-message phrase. A generation change detected while opening therefore escaped as a terminal unsafe-sidecar failure.

## Desired outcome

Represent legitimate sidecar generation changes as one explicit typed handoff outcome and retry them consistently during lock acquisition and inspection. Preserve the directory protocol because its non-empty removal interlock prevents a stale cleaner from deleting a replacement owner. Keep symlinks, non-directories, and other unsafe filesystem states fail-closed.

## Acceptance criteria

- Generation changes while opening or accessing a lease sidecar use one typed handoff classification.
- A creator that loses its uninitialized directory generation retries instead of terminating.
- Unsafe filesystem type and symlink replacements remain rejected or conservatively protected.
- Empty abandoned directories remain reclaimable without extending timing allowances.
- The separate-process archive-cache regression and full smoke aggregate pass.

## Rejected design

An initialized file published by hard link was evaluated and rejected: portable Node filesystems do not provide atomic conditional unlink by inode, so stale cleanup could unlink a replacement owner and permit concurrent lock holders.

## AI assistance

GPT-5.6 Sol via OpenCode analyzed the failed aggregate, evaluated and rejected an unsafe hard-link design during review, and drafted the typed handoff repair under Chris Huber's direction.

Beginne bei tryAcquireCacheLock() und verfolge die Pfade zum Öffnen und Zugreifen auf den Lease-Sidecar. Konzentriere dich darauf, wo Änderungen der Generation klassifiziert werden. Führe zuerst die Regression für den Archiv-Cache in separaten Prozessen aus, danach das vollständige Smoke-Aggregat. Als erledigt gilt die Änderung, wenn legitime Übergaben erneut versucht werden, während unsichere Dateisystemzustände weiterhin abgelehnt werden und verlassene leere Verzeichnisse weiterhin zurückgewonnen werden können.

Vom Indexierungsmodell aus dem Issue-Text verfasst.