fix(diskcache): make Close terminal

[leecha]

Problem

Operations arriving after Close could reopen the WAL because a missing writer was treated as a recoverable rotation state. This could allow stale cache instances to write after ownership changed and leave file descriptors open.

Fix

• Add a terminal closed state and ErrClosed.
• Make Close wait for in-flight operations.
• Close all resources before releasing the directory lock.
• Keep the lock file to ensure safe ownership handoff.
• Preserve writer recovery after rotation failures.
• Add lifecycle, ownership, resource cleanup, and race regression tests.

Validation

• go test ./diskcache
• Targeted race tests
• Windows cross-compilation