[MayCXC]

On an IVF column with quantizer=int8 or quantizer=binary, a point query reads past the end of the vector it returns, and a full scan or a KNN query returns the quantized bytes as the column's value.

create virtual table v using vec0(a float[16] indexed by ivf(nlist=2, quantizer=int8));
-- 10 inserts
select a from v where rowid = 3;   -- 64 bytes read from a 16-byte vector (ASan: heap-buffer-overflow in vec0Column_point)
select rowid, a from v;            -- 16-byte blobs, the int8 form of each float[16]
select rowid, a from v where a match :q and k = 10;  -- the same

A quantized column keeps each vector's int8 or binary form in its cell and the vector as it was inserted in _ivf_vectors, but ivf_get_vector_data() returned the cell's bytes, ivf_vec_size() of them, for every column. A quantized column's vectors are now read from _ivf_vectors, through a cached statement like the rowid-map lookup's, which is reset as soon as its row is copied out so a read-only query leaves no statement in progress.

Built with -DSQLITE_VEC_EXPERIMENTAL_IVF_ENABLE=1, on main none of 10 int8 or binary rows reads back as inserted from a point query, a full scan or a KNN query, before or after compute-centroids; here all 10 do from each, and an unquantized column reads back as before. Test: test_ivf_quantized_vectors_read_back in test-ivf-quantization.py, parametrized over int8 and binary; both fail on main and pass here. The Python suite passes (with the IVF define, 303 passed, 4 skipped; without it, 239 passed, 68 skipped), under ASan and UBSan as well, and both builds' warnings are main's.

#313 fixes the quantized paths of clear-centroids, set-centroid and assign-vectors and leaves this one as it is. This merges cleanly with it and with every other open PR that touches the IVF code, #343 and #347 included.

Origin. Found in the IVF port to photostructure/sqlite-vec, photostructure#12 (65817f4), which lists it among the port's differences from upstream; the change and its test are written here for asg017's main. No open or closed PR or issue here makes the same change.