List families similar to a seed family

Return up to 20 families similar to the seed family, ranked by the version-stamped similarity model and scoped to the workspace. The seed family is resolved like the family GET (a cross-firm or missing seed returns 404). limit is 1-20 (default 10) and is rejected, never clamped, when out of range; the response is not paginated (next_cursor is always null). When the seed has no similar families, the response is an empty list with 200 (not 404).