fix: honor radius/linear/refiner set before set*Params() (closes #197)

[s2x]

Description

Closes #197

Calling setRadius(), setLinear() or setUsingRefiner() on a ZVecVectorQuery before any set*Params() method silently dropped those settings — param setters replaced query_params_ wholesale, so queries ran with wrong settings (e.g. radius 0.0) with no error. Affected HNSW, IVF, Vamana, RaBitQ and Flat indexes.

Changes

• ffi/zvec_ffi.cc: new merge_stored_query_settings() helper applied after each of the 5 param setters (set_hnsw_ef, set_hnsw_rabitq_ef, set_vamana_ef_search, set_ivf_nprobe, set_flat_mode) — stored radius/linear/refiner values are merged into the freshly created params, so setter order no longer matters. Unconditional merge is a no-op when nothing was pre-set (constructor defaults 0.0f/false/false).
• tests/test_query_params_order.phpt: regression test (Flat + HNSW, L2) verifying radius/linear/refiner-before-params behave identically to params-first. Verified to FAIL pre-fix and PASS post-fix.
• CHANGELOG.md: Fixed entry.

Testing

• Builds locally (FFI libzvec_ffi.dylib rebuilt on Darwin)
• All .phpt tests pass: 181 tests — 177 passed, 2 skipped (RaBitQ Linux-only etc.), 2 expected-fail, 0 failed
• No test database leftovers (test_dbs/ empty)

Code Review

• Passed subagent code review (no correctness issues; CHANGELOG gap addressed)
• All review comments addressed