feat: add scoped durable memory management

[AetherAI3]

Summary

• add public vector-free MemoryRecord / MemoryPage DTOs and session-scoped page/get/replace/delete/clear methods
• enforce optimistic versions, immutable provenance, UUID memory IDs, and strict namespace isolation (including removing separate-mode global recall fallback)
• serialize pool access through one re-entrant lock, durably fsync successful mutations, and zero reclaimed mmap rows for float32/q4/q8 pools
• preserve legacy retrieval metadata by returning detached search snapshots

Security and durability

• foreign memory IDs resolve as scoped not-found errors
• caller tags and metadata patches cannot spoof source, versions, timestamps, or private chain coordinates
• clear is atomic on expected_count; replace/delete use compare-and-swap expected_version
• durable mutations flush vector data and atomically replace/fsync the sidecar before returning

Validation

• py -3.11 -m pytest -q: 563 passed, 5 skipped
• focused memory/storage suite: 40 passed, 3 skipped
• compileall and git diff --check: passed

All tests use isolated pytest directories; no live pool data was touched.