FFI: verify MultiQuery against v0.6.0 structure

[s2x]

Phase 1 — verification

v0.6.0 MultiQuery/SubQuery:

struct SubQuery {
  QueryTarget target_;
  int num_candidates_{10};
};
struct MultiQuery {
  std::vector<SubQuery> queries;
  int topk{10};
  std::string filter;
  bool include_vector{false};
  bool include_doc_id_{false};
  std::optional<std::vector<std::string>> output_fields_;
};

Tasks

• Check our queryMulti FFI path compiles and maps correctly to SubQuery.target_
• Rerank interface was switched to vector-based in v0.5.0 (#267) — our rerankers are pure PHP (no FFI), confirm no impact
• tests/ covering queryMulti pass

Depends on: #170, #171.

[s2x]

Completed in PR #184 (commits 85ad96c + 0c49c82, merged 2026-07-30). Verified on branch feat/issue-171-searchquery-migration: zero stale zvec::VectorQuery references in ffi/, SearchQuery+ in use across all query entry points, QueryParams::set_type() not used (removed upstream), FFI builds clean against zvec v0.6.0 and the query test suite passes (incl. output_fields semantics, groupby, vamana, multivector RRF).