-- pg_turbovec v1.27.1 -- Phase Q-4a: parallelize the IVF k-means build.
-- Build-SPEED change only. NO shippable behaviour change to a built
-- index: the persisted IVF centroids + assignment are BYTE-IDENTICAL
-- to v1.27.0 for a fixed (corpus, seed, lists, dim).
--
-- This file is a *reference mirror*. The authoritative install script
-- is generated by `cargo pgrx schema`.
--
-- NO wire-format change (stays VERSION 7), NO SQL-surface change, NO
-- REINDEX. Existing indexes are unaffected; this only makes NEW
-- IVF (WITH (lists = N)) builds faster.
--
-- What changed: two remaining serial hot loops in the k-means build
-- were parallelized bit-identically (the rest was already parallel
-- from v1.20.0/v1.22.1):
--   * gemm_lloyd_assign's per-row nearest-centroid argmin + top-2
--     tie-break -- run every Lloyd iteration over the whole sample --
--     is now a data-parallel par_iter_mut().enumerate() writing each
--     assign[i] to a fixed index (byte-identical regardless of thread
--     count).
--   * rotate_corpus_into's GEMM -- Parallelism::None -> Rayon(0), on
--     the same gemm-tiling-never-reduces-across-threads guarantee
--     v1.22.1 established for the Lloyd cross-term GEMM.
-- Left serial (deliberately): the k-means++ next-seed CDF pick
-- (sequential by construction) and the empty-cell reseed scan.
--
-- Measured ~1.91x build speedup (lists=4096, n=131072, dim=256,
-- 8-core, release) with bit-identical output. Sub-linear because
-- Lloyd iterations are sequentially dependent (only within-iteration
-- work parallelizes). Bit-identity across pool sizes {1,2,auto} is
-- asserted by ivf_coarse_model_bit_identical_across_pool_sizes +
-- rotate_corpus_bit_identical_across_pool_sizes.
--
-- Migration: `ALTER EXTENSION pg_turbovec UPDATE TO '1.27.1';`. No
-- REINDEX -- a no-op upgrade for existing indexes; new IVF builds are
-- just faster.