feat: banded admission for the per-subject embedding store

AR-018 — an embedding joins a track's store only if its similarity to something
already there falls inside a band, rather than merely being far from the gallery.

Above the upper bound it is redundant: another look at a pose the store already
covers, teaching the annex nothing while costing a slot a novel view could have
used. Below the lower bound it is suspect: within one track every face is the
same person by construction, so an embedding unlike everything else on the track
is evidence that construction failed — a track-ID collision or a bad detection.
Admitting it is exactly how an actor's annex gets poisoned with someone else's
face.

The old gate had only the upper half of that idea, expressed as a raw cosine
against the gallery. Both bounds are now calibrated probabilities (AR-024), so
the same number means the same thing here as in association and evidence
weighting rather than three different things.

This catches track-ID collisions EARLIER than the spread gate did — at the door
rather than at promotion — so the buffer never becomes two-person in the first
place. The spread gate stays as a second line for a track that drifts gradually
instead of jumping. The existing test was asserting the mechanism rather than
the outcome, so it was rewritten to assert what actually matters: whichever gate
fires, the outsider must not reach the annex.

Rejections are counted. A store that admits nothing is as broken as one that
admits everything, and neither is visible otherwise.

Band defaults 0.90-0.95 are working values pending VR-007; the two bounds fail in
opposite directions and must be swept separately.

Suite: 92 cases, 6133 assertions.

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

TRACES: AR-018, AR-024 | SR-005

src/nodes/identity_matcher_node.hpp

@@ -106,6 +106,12 @@ struct IdentityMatcherFunc {
                         flat_emb_[i].data(), 512 * sizeof(float));
 
         sim_engine_ = make_similarity_engine(host_gallery.data(), n_gallery_, kMaxFaces);
+
+        /// TRACES: AR-018, AR-024 | SR-005
+        // The expansion store thresholds in the same probability space as
+        // association and evidence weighting, so a "0.9" means one thing
+        // pipeline-wide rather than three.
+        track_gallery_.set_calibration(same_person_probability(cal_));
     }
 
     /// TRACES: AR-023, AR-024 | SR-002