Add VectorSearchEngine backed by turbovec (speedkick) - #1599

Purpose of the change

A VectorSearchEngine over turbovec: an in-memory index of 4-bit TurboQuant codes.

It scans every vector, like the engines beside it, so it changes no asymptotics. What it changes is the memory traffic an exhaustive scan is bound by, which 4-bit codes cut about eightfold.

Why it lands after #1598

Under the engine contract before #1598 this engine had to supply get_vectors, and it could only ever raise: turbovec keeps quantized codes and does not retain the originals. Adding a method that exists to refuse, and retiring it a PR later, is churn that says nothing. On the contract #1598 left behind there is no keyed vector access to refuse, so every method this engine declares, it implements.

Description

• Scores are clamped onto [-1, 1]: quantization can carry an inner product slightly outside it, and a similarity reading 1.0000001 is a lie about the range the contract promises.
• Vectors are zero-padded to a multiple of eight — the width the index can hold — and normalized on the way in, so the inner product it computes is cosine.
• turbovec has a native allowlist search, but it needs the allowed ids enumerated, and the engine contract's allowed_keys only answers membership. So the engine fetches unrestricted, drops what the filter rejects, and widens the fetch until limit survive or the whole index has been scanned, as usearch does. Handing turbovec the ids directly waits for the contract to carry them (DEFERRED (until lifecycle/DDL changes): Route filtered queries by selectivity, and score allowlists exactly (speedkick) #1602, deferred).
• Like the engines beside it since Own the search engine's concurrency in the store, not in each engine (speedkick) #1612, it holds no lock of its own: the store serializes access.
• turbovec>=1.0.0 as an optional extra; 1.0.0 is upstream's first stable release and commits to the on-disk format this persists through.

Verification

• pytest packages/server/server_tests: 1963 passed, 3 skipped.
• ty check --project packages/server: clean.
• ruff check / ruff format --check / uv lock --check: clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NKmF9xNph9QH3ozNw3ZnJL