Define Hybrid Retrieval

The reference now exposes lexical and semantic retrieval with scores, but assistants still have to call both endpoints and merge results client-side. A server-side hybrid endpoint would make the recall layer simpler, safer, and easier to evaluate.

Expose a separate endpoint rather than changing lexical retrieval:

PDPP SHALL define an optional experimental hybrid-retrieval extension for implementations that combine lexical and semantic retrieval into one result list. Clients SHALL NOT assume the extension exists unless resource-server metadata advertises it.

GET /v1/search SHALL remain the lexical retrieval endpoint even when a server also advertises hybrid retrieval. Hybrid retrieval SHALL NOT silently alter lexical result ranking, filtering, scoring, or response shape.

GET /v1/search/semantic SHALL remain the semantic retrieval endpoint even when a server also advertises hybrid retrieval. Hybrid retrieval SHALL NOT silently alter semantic result ranking, filtering, scoring, model identity, or response shape.