PS-11266: Basic HNSW class (phase 1 - basic modification and read operations support).

[dlenev]

https://perconadev.atlassian.net/browse/PS-11266

Added basic implementation of in-memory HNSW index.

It supports insert and approximate k-NN search per the HNSW paper
(https://arxiv.org/abs/1603.09320) and uses arena allocator for nodes.

Streaming approximate NN search is also supported.

Added unit tests.

[Copilot]

Pull request overview

Adds an arena-backed, in-memory HNSW index with approximate nearest-neighbor search.

Changes:

• Implements insertion, k-NN, streaming search, and validation.
• Adds unit and recall tests.
• Registers the new test target.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
vector-common/hnsw.h	Implements HNSW indexing and search.
unittest/gunit/hnsw-t.cc	Tests index and streaming behavior.
unittest/gunit/CMakeLists.txt	Registers HNSW tests.

Suppressed comments (1)

vector-common/hnsw.h:553

• The allocator contract permits nullptr, and MEM_ROOT::Alloc() does return it on OOM. With NDEBUG, this assertion vanishes and placement-new at address zero is undefined behavior. Propagate allocation failure before constructing the node.

      assert(raw_mem != nullptr);

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[catalinbp]

lgtm with the exception of the m_visited which needs to be properly released in reset()