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.