Minor v0.5.1 options: search prefetch (PO/PL), mmap COW, is_dirty

[s2x]

Phase 3 — nice-to-have knobs from zvec v0.5.1

• Search prefetch configuration — pass PO/PL through search params (#482)
• Storage MMAP options — copy-on-write mmap config, MMAP_POPULATE fix (#480) → candidate for ZVecCollectionOptions
• Index dirty status — is_dirty on core Index interface (#488) → could surface in ZVecCollectionStats

Tasks

• Investigate upstream exposure, decide per-item worth
• Implement the ones that map cleanly to PHP API
• Tests + docs

Depends on: #170.