-- pg_turbovec v1.21.0 -- Phase G-1: centroid graph for IVF coarse-cell
-- selection.
--
-- This file is a *reference mirror*. The authoritative install
-- script is generated by `cargo pgrx schema`.
--
-- v1.21.0 lands a small in-memory Vamana/HNSW-lite graph over the
-- IVF coarse centroids (, Phase G-1). All
-- changes are scan-path / in-memory only -- NO wire-format change
-- (MetaPageData::version stays 5), one new GUC, and NO REINDEX.
--
--   * Centroid graph coarse-cell selection: for an out-of-core IVF
--     index (turbovec.out_of_core cell-scoped path) with lists >=
--     4096, coarse_probe can now navigate a small fixed-out-degree
--     (16) undirected graph over the centroids instead of scoring
--     every centroid -- O(lists*dim) -> greedy graph search. The
--     graph is built ONCE PER BACKEND, IN-MEMORY, from the
--     already-persisted coarse centroids (a deterministic function
--     of them, symmetrized so greedy search is recall-safe), so
--     nothing new is persisted and EXISTING v4/v5 IVF indexes get
--     it for free on the next scan -- NO REINDEX.
--
--   * turbovec.coarse_graph (NEW GUC, enum, default 'auto' = build
--     the graph only when lists >= 4096; 'on' forces it regardless
--     of lists; 'off' always uses the exact linear scan).
--
-- No DDL beyond the new GUC (cargo pgrx schema emits it). No on-disk
-- migration. `ALTER EXTENSION pg_turbovec UPDATE TO '1.21.0';` is
-- sufficient. Existing indexes are unaffected and benefit from the
-- centroid graph (when lists >= 4096) with no rebuild.