refactor: extract total_count_ to InnerIndexInterface base class

[LHT129]

Summary

Move total_count_ from HGraph, BruteForce, and WARP into the InnerIndexInterface base class as std::atomic<uint64_t>.

Changes

• Add std::atomic<uint64_t> total_count_{0} to the protected section of InnerIndexInterface
• Remove the per-index total_count_ member from HGraph, BruteForce, and WARP
• Update BruteForce and WARP to use atomic operations (.load(), .store(), prefix ++/--) for all total_count_ accesses
• HGraph was already using std::atomic<uint64_t> so its usage is unchanged

Motivation

• Eliminates code duplication across index implementations
• Ensures thread-safe access uniformly (BruteForce and WARP previously used non-atomic uint64_t)
• Provides a consistent base for future index implementations

Supersedes #1682 (closed due to directory restructure in #2127).
Closes: #1681

[mergify]

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[gemini-code-assist]

Code Review

This pull request refactors total_count_ into a shared atomic member variable in InnerIndexInterface, updating its usage across the BruteForce and WARP implementations. The review feedback identifies critical race conditions in the Add methods of both algorithms, where total_count_ is incremented before data insertion is complete, potentially exposing uninitialized data to concurrent searches. Additionally, the reviewer recommends caching total_count_.load() in WARP search and range search methods to prevent inconsistencies and reduce atomic load overhead.

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