feat: identity evidence reaches the registry

Closes the link that made AR-012 inert: the tracker was maintaining registry
state, but nothing called observe(), so no belief accumulated, no track was ever
owned, and no presence claim could be emitted. Tracking worked and presence did
not.

The matcher now feeds every scored face to the registry as a calibrated
posterior plus its embedding. Deliberately every scored face, not only the ones
clearing prob_threshold: a run of near-misses for one actor is evidence, and
discarding it would leave ownership depending on the per-frame threshold this
redesign exists to stop relying on. The registry discounts for correlation and
decides ownership from the accumulated posterior (AR-025).

The registry is an optional dependency of the matcher. Without one it behaves
exactly as before, which keeps the replay harness and the unit tests working
unchanged rather than forcing every caller to construct a registry it does not
need.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-012, AR-025 | SR-002

src/main.cpp

@@ -206,6 +206,8 @@ int main(int argc, char** argv) {
     auto registry = std::make_shared<TrackRegistry>(
         reg_cfg, EvidenceDiscounter(same_person));
 
+    matcher_fn.set_registry(registry);
+
     FaceTrackerFunc                   ftracker_fn{cfg, registry, same_person};
     SceneTrackerFunc                  tracker_fn {cfg};
     ResultSinkFunc                    sink_fn    {cfg, done};