Semantic entity search within a space's graph

Semantic search over the entities in a space. JSON body: text (1-2000 chars, required — the natural-language query), optional topK (1-100) to cap results, optional types (non-empty array of organization | person | location | identifier | date | amount | other) to restrict to entity kinds. Returns { matches }, each { id, type, name, score } ranked by similarity (higher score = closer). Requires space:read on :spaceId; missing access surfaces as a 404.