feat: GroupByVectorQuery cannot set index-specific query params (ef/nprobe)

[s2x]

Feature gap (found during #197 review, verified by subagent)

ZVecGroupByVectorQuery cannot set index-specific parameters:

• ZVecGroupByVectorQuery::setHnswParams(), setFlatParams(), etc. all throw ZVecException ("not directly supported") — src/ZVecGroupByVectorQuery.php:210-247
• No group-by param setters exist in the FFI layer (ffi/zvec_ffi.h:319-329)
• The group-by ensure path (ffi/zvec_ffi.cc:2318) only creates params when none exist

Impact

No way to tune ef/nprobe for group-by vector queries — users stuck with defaults. Low priority since group-by queries still work with defaults.

Suggestions

• Check whether upstream zvec C++ supports group-by query params (zvec/src/include/zvec/db/query.h — GroupByVectorQuery params).
• If yes: add FFI setters + PHP fluent methods mirroring ZVecVectorQuery.