-- 2.6.0 — 1-bit sign binary quantization (bit_width = 1) end to end.
--
-- WITH (bit_width = 1) now builds, scans, inserts and vacuums. Previously the
-- reloption was accepted for forward compatibility but the build raised a
-- clear "not yet implemented" ERROR.
--
-- 1-bit is NOT TurboQuant-at-1-bit: the turbovec crate hard-rejects
-- bit_width < 2 in both constructors, so this is a distinct scheme -- sign
-- binary quantization (the DiskANN/pgvector/Qdrant coarse code). Per-vector
-- storage is dim/8 with NO scale (half the 2-bit stride), scored by Hamming
-- (popcount of XOR) and then exactly reranked by the AM.
--
-- NO wire-format change (stays v8) and NO REINDEX: this is a new kind byte
-- (KIND_BQ = 3), not a version bump. Every existing index keeps
-- kind = SINGLE/COLBERT/GRAPH and decodes byte-identically -- the same
-- additive per-kind path v4->v5->v6 used. The three bq_mean_* meta fields
-- occupy page offset 316, which was reserved-and-zero on every prior version,
-- so an old meta page reads them as absent.
--
-- Mean-centering is load-bearing: the naive sign-at-zero rule sets every bit
-- on dense-positive data (measured R@10 = 0.0 on GIST), so the per-dim corpus
-- mean is subtracted before taking signs, persisted, and applied to queries.
-- A corpus still collapsed AFTER centering (constant/near-constant) is
-- rejected at build rather than shipping a signal-free index.
--
-- Not yet composed: bit_width = 1 with lists > 0 (IVF), rejected with a clear
-- ERROR. bit_width = 1 with graph = true stays rejected in the reloption
-- validator (and the graph kind is deprecated as of 2.5.0).
--
-- No SQL surface change. This migration is intentionally empty.