feat(buffer): harden shared buffer pool core

[iaojnh]

Summary

This PR hardens the shared Buffer Pool core for future Buffer Storage integration with IVF, DiskANN, and other indexes. Index integrations, benchmarks, and user documentation are out of scope.

Key changes

• Unify page buffers, metadata, and external caches under one process-wide memory budget.
• Harden concurrent page loading, pinning, eviction, and failure recovery.
• Add admission control, hot-page protection, and priority-aware CLOCK eviction.
• Support dirty-page tracking, batched writeback, failure recovery, and file extension.
• Add background reclamation with low-watermark control.
• Share Buffer Pool state across executables and DSOs.
• Consolidate io_uring support and reuse its reader types in DiskANN.
• Add statistics for memory usage, hits, eviction, admission, and writeback.

Verification

• vector_page_table_test: 66/66 passed.
• Concurrent load-coalescing test: 2,000 repeated runs passed.
• CLOCK second-chance test: 1,000 repeated runs passed.
• clang-format 18 passed.
• MSVC and Linux Clang compatibility issues were fixed.