Use of the local cache and optional APCu

[Fraenkiman]

• fileio core
• entries core

Note for this development stage:

     Load scenario: 3,000 posts × 5 comments.
     Search (search.php) touches most entries and dominates memory.
     Peak calculation:
       fp:io:*
          3,000 × ~1.93 KiB + 15,000 × ~0.52 KiB ≈ 13.3 MiB
       fp:entry:parsed:* (TTL 600 s, e.g. after search)
          3,000 × ~1.98 KiB ≈ 6.0 MiB
       Plugins/Smarty/other: ~2–3 MiB
     Worst-case simultaneously ≈ 21–23 MiB < 32 MiB; headroom ≈ 9–11 MiB covers fragmentation.
     Note: APCu is a shared pool per FPM pool (apc.shm_size), not per child process.