-- pg_turbovec v1.20.0 -- IVF scaling: parallel build/scan + sublinear coarse.
--
-- This file is a *reference mirror*. The authoritative install
-- script is generated by `cargo pgrx schema`.
--
-- v1.20.0 lands the IVF scaling work surfaced by the benchmark A/B/C
-- benchmark. All changes are scan-path / build-path / in-memory --
-- NO wire-format change (MetaPageData::version stays 5), no SQL
-- surface change beyond one new GUC, and NO REINDEX.
--
--   * Sublinear two-level coarse quantizer: for an IVF index with
--     lists > 4096, cell selection is now O(sqrt lists) instead of
--     O(lists). The two-level structure is computed IN-MEMORY at
--     index-open from the already-persisted coarse centroids (a
--     deterministic function of them), so nothing new is persisted
--     and EXISTING v4/v5 IVF indexes get it for free on the next
--     scan -- NO REINDEX. Measured 39-364x fewer centroid distance
--     computations per query at recall 1.0, which lets `lists` grow
--     large (tiny cells) without the coarse step becoming the
--     bottleneck -- the key enabler for IVF at 10M+.
--
--   * turbovec.scan_parallelism (NEW GUC, int, default 0 = auto =
--     min(cores, 4); 1 = serial): parallelizes the per-query IVF
--     fine-scan across probed cells (out-of-core path), cutting
--     single-query latency at high dimension. Conservative default
--     to protect aggregate QPS under concurrency. Results are
--     identical to the serial scan (same top-k).
--
--   * Parallel IVF build: the k-means training and assign-sweep now
--     use the bounded build pool (turbovec.build_parallelism) across
--     cores, memory-bounded and BYTE-IDENTICAL (the reduction order
--     is a fixed function of the input, independent of thread count).
--     Modest speedup at high dim (the single-threaded GEMM remains
--     the dominant term); a deterministic parallel GEMM is scoped
--     follow-up work.
--
-- No DDL beyond the new GUC (cargo pgrx schema emits it). No on-disk
-- migration. `ALTER EXTENSION pg_turbovec UPDATE TO '1.20.0';` is
-- sufficient. Existing indexes are unaffected and benefit from the
-- sublinear coarse + parallel scan with no rebuild.