Pin the current behaviour of the vector candidate stage in
search_engine.cpp before it is extracted into search_vector_stage.{h,cpp}.
A new catch2_search_vector_stage binary runs the matrix policy
{Shadow, Augment, Narrow commit, Narrow fallback} x backend
{SimeonPqAdc, ExactScan, Vec0L2} with stage trace off and on, plus one
executor-backed fanout case, and compares each cell against explicit
goldens: fused hash/rank/score and vector contribution, raw vector
ComponentResults (trace_component_hits_json), the stage's debug keys
without timings, certificate.work / admission.work via the
topology_route_work_* and admission keys, and the merged vector_search_*
diagnostics. Quirks are pinned as-is: relaxed_vector_primary_hit_count is
last-writer, Vec0 keeps global results unchanged under Shadow/Augment,
Narrow commits only on Simeon PQ because ExactScan and Vec0 never observe
an ANN candidate budget (work_observation_unavailable), and Vec0 leaks a
partial ann_candidate_budget_actual into the unobserved work record. A
hidden [.dump] case regenerates the goldens.
Move the shared topology fixtures (TopologySearchFixture,
buildTwoClusterTopologyBatch, topologyRoutingTestConfig,
runTopologySearch, configureCertifiedTopologyRoute, TopologyVectorEnvPin)
into tests/unit/search/topology_search_test_support.h so both tests use
the same corpus. runTopologySearch takes an optional executor and
configureCertifiedTopologyRoute an options struct; existing call sites are
unchanged. The portability allowlist entry for the raw YAMS_TEST_TMPDIR
read moves with the helper. No product behaviour changes.
Signed-off-by: trvon <me@trevon.dev>
