feat(ar-024): enforce the invariant statically, and delete the fallback it caught

AR-024's register row gives its verification tier as "Static check -- no
bare cosine outside a tagged EXCEPTION". No such check existed, so the
invariant was enforced by reading, and reading had missed a live
violation.

scripts/ci/check_raw_cosine.py is that check, wired into the
traceability workflow as a blocking step. It is honest about its reach:
it catches direct cosine_similarity() uses not routed through a
calibration, and it cannot follow a cosine through a variable across
statements. That limit is documented in the script rather than left for
someone to discover after trusting a pass.

What it caught, and what this commit removes with it:

The identity matcher's no-calibration fallback thresholded raw cosine
distance (match_threshold) plus a ratio test (match_ratio,
match_ratio_ceil). Worse than the invariant breach: it fed
max(0, cosine) into TrackRegistry::observe, whose contract reads
"posterior is a calibrated probability, never a raw cosine (AR-024) ...
so the accumulation cannot be fed an uncalibrated number by a careless
caller". It could, and did. And it disagreed with the rest of the
pipeline about what "the fit failed" means -- same_person_probability
answers that with the untuned default sigmoid and a loud warning, so
association stayed in probability space while matching alone left it.
One run, two policies, no announcement.

Now one rule: cal_.probability() always, with a warning when the fit is
not real. A worse answer than a fitted calibration, a better one than a
number whose units nothing else shares.

TrackGallery::set_calibration is mandatory for the same reason. Its
default was max(0, cosine), which made expand_band_lo = 0.90 mean
"cosine > 0.9" in a test and "P(same person) > 0.9" in production.
FaceTrackerFunc already threw without one; the expansion store now
matches.

One exception is recorded, in the calibration's own dedup. It is not a
close call: at 1 - 1e-7 it asks whether two vectors are the same vector,
and it runs on the fit's input, so a calibrated comparison there would
have to be calibrated by the fit it is feeding.

Also drops seven dead keys from the optimizer's CFG_KEYS. Config keys
are read with a contains() check, so each one had been silently inert
since the field behind it was deleted -- a sweep varying one of them
measured nothing and reported an ordinary-looking F1.

TRACES: AR-024, AR-023 | SR-002

src/gallery/gallery_calibration.hpp

@@ -162,6 +162,19 @@ inline GalleryCalibration calibrate_gallery(
         for (const auto& e : by_actor[ai]) {
             bool dup = false;
             for (const auto& k : kept) {
+                // EXCEPTION: AR-024 this asks whether two vectors are THE SAME
+                // VECTOR, not whether two faces are the same person.
+                //
+                // Two independent reasons, either sufficient. First, at
+                // 1 - 1e-7 the threshold is a floating-point identity test: it
+                // catches one source image embedded twice, and no genuine pair
+                // of distinct photographs lands there. Nothing about it is a
+                // decision, so there is nothing for a probability to mean.
+                //
+                // Second, and structurally: this IS the calibration fit. The
+                // dedup runs on its input, before (a, b) exist. A calibrated
+                // comparison here would have to be calibrated by the fit it is
+                // feeding, which is not a thing that can be arranged.
                 if (cosine_similarity(e, k) > kDedupSimThreshold) { dup = true; break; }
             }
             if (!dup) kept.push_back(e);