How an index entered TableMeta.indexes. Auto indexes are synthesized
at CREATE TABLE time from PRIMARY KEY / UNIQUE constraints. UserDeclared
indexes come from CREATE INDEX DDL. The distinction drives surface
rendering (auto-indexes omitted from .schema), schema-rendering verbose
flags, and sync DDL emission (auto-indexes are not re-emitted since they
are derived from the CreateTable payload).
Direction for a column within an engine-local index declaration.
Distinct from contextdb_parser::ast::SortDirection, which carries a
CosineDistance variant that is meaningful only for vector ordering.