feat(buffer): integrate IVF and DiskANN with shared page cache

[iaojnh]

Summary

• Add Buffer Storage support for IVF and DiskANN.
• Reduce IVF posting-list copying through page-span reads.
• Cache DiskANN graph pages with batched direct-I/O fallback under cache pressure.
• Handle bypass-only budgets, preserve backing-file identity, and fix cache/I/O resource lifetimes.

Existing mmap and direct-I/O modes remain available.

Testing

• macOS: 298 tests and 920 targeted regression executions passed.
• clang-format and clang-tidy passed.
• Linux and Windows validation pending CI.