supports
Whether this particular instance can actually run vector search. Implementing the interface is a type-level promise; this is the runtime truth. A store that can be wired without an embedder (so it satisfies the type but has nothing to embed with) should override this to report whether it was given one. Lets a caller — e.g. PropositionStoreTemplate.supportsVector — distinguish "configured for vectors" from "vector search will quietly return empty".