Add vexdb_lite extension

[vibeinging]

This PR adds vexdb_lite, an HNSW-based vector search extension for DuckDB.

Features

• Native FLOAT[N] vector type (no custom type required)
• Distance functions: L2 / Cosine / Inner Product (operator aliases <->, <=>, <#>, <~>)
• Graph (HNSW) ANN index via CREATE INDEX ... USING GRAPH_INDEX
• Product Quantization (PQ) for compact indexes
• Query optimizer rewrite: ORDER BY distance(...) LIMIT k → index scan
• Runtime SIMD dispatch (SSE / AVX2 / AVX-512 / NEON)

Details

• Source: VexDB-THU/VexDB-Lite @ v0.0.15 (MIT)
• Dependencies: only header-only boost/preprocessor (provided via vcpkg boost-preprocessor); no BLAS/Fortran/OpenMP.
• Platforms: linux/osx amd64 + arm64. Windows/WebAssembly excluded for now (not yet built/validated there).
• Standard loadable extension (cmake + duckdb_extension_load), no DuckDB core changes.

Quickstart

CREATE TABLE items (id INTEGER, vec FLOAT[3]);
INSERT INTO items VALUES (1, [1.0, 2.0, 3.0]), (2, [4.0, 5.0, 6.0]), (3, [1.0, 2.0, 4.0]);
CREATE INDEX idx ON items USING GRAPH_INDEX (vec) WITH (metric = 'l2');
SELECT id FROM items ORDER BY array_distance(vec, [1.0, 2.0, 3.0]::FLOAT[3]) LIMIT 2;