Upgrade to zvec v0.6.0 — FFI + ext migration (closes #170 #171 #172 #173 #174)

[s2x]

What

Upgrade the upstream alibaba/zvec dependency from v0.4.0 → v0.6.0 and migrate the C++ wrappers (FFI shared library + PHP extension) to the v0.6.0 C++ API. All breaking changes are confined to the C++ layer — the PHP-facing C API in ffi/*.h is byte-identical and src/*.php is untouched (one defensive guard in toCStringArray), so the public PHP API stays backward compatible.

Closes #170, #171, #172, #173, #174, #32.

Commits

1. 8f2e377 — bump submodule to v0.6.0 + fix link list (libzvec_db.a→libzvec.a, add libsnowball.a+libutf8proc.a, DiskANN now built-in / libaio via dlopen)
2. 85ad96c — migrate ffi/zvec_ffi.cc and php-ext/zvec_collection.cc to the v0.6.0 query structs
3. 0c49c82 — completeness: bump embedded version constants v0.4.0→v0.6.0, remaining v0.4.0 refs in workflows/docs, defensive toCStringArray guard, regression test for FFI: output_fields became std::optional — empty vector now means 'select no fields' #172

Per-issue verification (rigorous, done after the 3rd commit)

Issue	Checklist item	Status	Evidence
#170	zvec lib builds at v0.6.0	✅ DONE	zvec/build/.zvec_version = v0.6.0; libzvec.a 12.7MB present
#170	link list correct (new externals)	✅ DONE	libsnowball.a + libutf8proc.a added; every referenced .a exists in build (full set compared: 41 produced libs, all refs resolved)
#170	diskann plugin .so gone, libaio dlopen	✅ DONE	libcore_knn_diskann.a built-in, no plugin
#170	default version strings bumped	✅ DONE	grep v0.4.0 across repo (excl. CHANGELOG/MIGRATION history) → 0 hits in build/docs/workflows; ffi/zvec_ffi.cc version constants = v0.6.0
#171	VectorQuery→SearchQuery+QueryTarget	✅ DONE	rg VectorQuery in .cc → only VectorQueryHolder (our holder name, wraps SearchQuery query); target_.field_name_, target_.set_vector(), target_.query_params_ everywhere
#171	all query entry points migrated	✅ DONE	fp32/fp16/fp64/sparse/queryById/queryByFilter — both ffi & ext
#171	QueryParams::set_type() unused	✅ DONE	rg set_type in ffi/ php-ext/ → 0 hits
#172	leave nullopt when no output fields	✅ DONE	rg output_fields_ = {} / empty vector assign → 0 hits; setters only touch it when count>0
#172	regression test added	✅ DONE	tests/test_query_output_fields_semantics.phpt (added in commit 3) — passes
#173	group_topk_→topk_per_group_	✅ DONE	rg group_topk_ → 0 hits; topk_per_group_ used
#173	field/vector/params into target_	✅ DONE	confirmed in GroupByVectorQueryHolder
#173	test_groupby_query_object.phpt passes	✅ DONE	green
#174	MultiQuery path verified	✅ DONE	no C-side MultiQuery (PHP does per-field SearchQuery + rerank); rerank purely PHP (rg Rerank in ffi → 0)

Build & test results

• ./build_ffi.sh → ffi/build/libzvec_ffi.dylib (53MB) links clean
• ./php-ext/build_ext.sh → php-ext/modules/zvec.so (32MB) builds & loads
• php -n run-tests.php -q tests/ → 171 passed, 2 XFAIL, 0 failed (175 total)

Side effects

• bug_0002 now passes — zvec v0.6.0 implements group-by; --XFAIL-- removed
• test_extended_query_params expectation: 1 group → 2 groups (v0.6.0 returns even/odd correctly)
• FP64 XFAIL comments updated v0.4.0→v0.6.0 (still unsupported upstream — separate issue)

Out of scope (separate issues, v0.6.0 milestone)

New v0.6.0 features not included — only the migration: #176 fetch outputFields, #177 random rotation, #178 includeDocId, #179 DiskAnnIndexParams split, #180 FTS, #181 minor v0.5.1 options.

Known non-blocker

Running the suite with the native zvec extension loaded fails test_vector_query_object.phpt on ZVecVectorQuery::setTopk() — a pre-existing native-ext feature gap, unrelated to this upgrade. FFI mode (CI path) is green.