refactor: Consolidate duplicate LinearPool/SearchBuffer and Bitset implementations

[huanglune]

Summary

There are duplicate implementations for search buffer and visited set structures across the codebase that should be consolidated.

Duplicate Implementations

1. LinearPool vs SearchBuffer

Feature	include/utils/query_utils.hpp::LinearPool	include/utils/rabitq_utils/search_utils/buffer.hpp::SearchBuffer
Purpose	Sorted linear buffer for graph-based ANN search	Sorted linear buffer for graph-based ANN search
Binary search	find_bsearch()	binary_search()
Insert method	insert() with memmove	insert() with memmove
Pop method	pop() with set_checked	pop() with set_checked
Checked flag	Uses top bit (bit 31)	Uses top bit (bit 31)
has_next	✅	✅
is_full	✅	✅

Both classes use the same algorithm and design pattern, with minor implementation differences.

2. DynamicBitset vs HashBasedBooleanSet

Feature	include/utils/query_utils.hpp::DynamicBitset	include/utils/rabitq_utils/search_utils/hashset.hpp::HashBasedBooleanSet
Purpose	Track visited vertices	Track visited vertices
Implementation	Bit vector (uint64_t array)	Hash table + unordered_set fallback
Interface	set(), get(), reset()	set(), get(), clear()

Both serve the same purpose of tracking visited nodes during graph search.

Suggested Refactoring

1. Unify SearchBuffer/LinearPool: Keep one implementation (suggest SearchBuffer as it's cleaner) and deprecate the other
2. Create a unified visited set interface: Abstract the visited tracking with a common interface, allowing different implementations (bitset for dense, hash for sparse) based on use case
3. Move shared utilities to a common location: The set_checked/is_checked pattern using bit 31 could be a shared utility

Files Involved

• include/utils/query_utils.hpp
• include/utils/rabitq_utils/search_utils/buffer.hpp
• include/utils/rabitq_utils/search_utils/hashset.hpp

Benefits

• Reduced code duplication and maintenance burden
• Consistent behavior across different index types
• Easier to optimize and fix bugs in one place