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/track_gallery.hpp

@@ -9,6 +9,7 @@
 #include <iostream>
 #include <limits>
 #include <map>
+#include <stdexcept>
 #include <string>
 #include <vector>
 
@@ -163,10 +164,20 @@ struct TrackGallery {
     }
 
     /// TRACES: AR-024 | SR-005
-    /// Supply the calibration belonging to the active embedder. Without it the
-    /// band falls back to treating cosine as probability, which is wrong but
-    /// bounded — and the default is loud in the header rather than silent.
-    void set_calibration(std::function<float(float)> c) { calibrate_ = std::move(c); }
+    /// Supply the calibration belonging to the active embedder.
+    ///
+    /// Required, not optional. The default used to be `max(0, cosine)` — a raw
+    /// cosine worn as a probability, which made `expand_band_lo = 0.90` mean
+    /// "cosine above 0.9" in a test and "P(same person) above 0.9" in
+    /// production. Those are wildly different gates, and nothing announced the
+    /// switch. `FaceTrackerFunc` already refuses to construct without a
+    /// calibration for the same reason; this now matches it.
+    void set_calibration(std::function<float(float)> c) {
+        if (!c) throw std::invalid_argument(
+            "track_gallery: a calibration is required — the admission band is "
+            "expressed in probability space (AR-024)");
+        calibrate_ = std::move(c);
+    }
 
     /// Embeddings the band refused. A store that admits nothing is as wrong as
     /// one that admits everything, and neither is visible without this.
@@ -341,8 +352,9 @@ private:
     }
 
     /// cosine → P(same person). The one probability space the pipeline reasons
-    /// in; see gallery_calibration.hpp's same_person_probability.
-    std::function<float(float)> calibrate_{[](float c) { return std::max(0.f, c); }};
+    /// in; see gallery_calibration.hpp's same_person_probability. Never default
+    /// constructed to an identity-ish stand-in — see set_calibration.
+    std::function<float(float)> calibrate_;
     std::size_t             rejected_{0};   ///< admissions refused by the band
 
     bool                    enabled_;