refactor(bench): SuperHero replaces Road to Bali as the reference film

Bali was chosen because the TRECVID DVU set ships character mugshots, but
its reference crops are unusable at scale: median detected face 27 px
against a 69 px maximum, so every reference was upscaled 4x or more past
what the embedder was trained for (AR-011). A 66 px floor left 2 of 69
references; no threshold exists that both keeps the faces in distribution
and leaves enough of them to calibrate.

SuperHero is 69 px median and 241 px max. Its gallery builds at a 66 px
floor with 14 references over 5 characters, and calibrates on its own
(a=15.2867 b=-4.98633, 100% train accuracy) instead of borrowing constants.

Measured on the fused 17-minute film, one stream rather than per-scene
clips so presence windows cross real scene boundaries as SR-002 intends:
precision 1.00, recall 0.65, F1 0.79 — 13 true positives, 0 false
positives, 7 misses. Every out-of-gallery character was declined rather
than forced onto a nearest match. The misses are the short scenes (14 s,
38 s, 27 s), consistent with per-track accumulation needing sightings.

- build_gallery gains --min-face-px, filtering the *detected face* rather
  than the crop. The DVU images are scene crops, not mugshots, so crop
  dimensions say nothing about face scale. A poisoned reference is
  permanent in a way a bad frame is not: it corrupts every future match
  against that identity.
- scripts/fetch_dvu.sh fetches mugshots, scene graphs and segmentation for
  any DVU film. NIST names the same film three different ways, so KG_DIR
  and KG_FILE are overridable rather than derived. This exists as a script
  because the first copy of this data was assembled ad hoc in /tmp and was
  lost with it, taking the working gallery along.
- Replay fixtures move to the artifact registry: push/pull_artifacts.sh
  gain a replay-fixtures target, and tests/fixtures/dumps/.gitignore keeps
  them out of git. superhero.h5 is ~9 MB and regenerating it needs the
  film, the models and a GPU — none of which CI has. The gallery ships
  with the dumps, since a dump only replays against the gallery it was
  produced with.
- AR-012 and AR-013 coverage is ported onto the new fixture rather than
  dropped with the Bali cases: 12369 assertions, up from 7991, since the
  film is an order of magnitude larger than the clips.

Suite: 15679 assertions, 101 test cases.

TRACES: AR-011, AR-012, AR-013 | VR-001, VR-005 | SR-002

src/gallery/gallery_builder.cpp

@@ -96,6 +96,27 @@ ActorGallery build_gallery(const BuildConfig& cfg) {
                     return a.confidence < b.confidence;
                 });
 
+            // Reject faces too small to embed honestly.
+            //
+            // The reference images are crops cut from the film, not mugshots, so
+            // the detected face can be a small fraction of the image. Upscaling a
+            // 30 px face to ArcFace's 112x112 feeds the model an input it was
+            // never trained for, and it answers with a confident, plausible,
+            // wrong embedding.
+            //
+            // At inference that costs one frame. Here it is permanent: a poisoned
+            // reference sits in the gallery and corrupts every future match
+            // against that character, which is exactly the kind of error that is
+            // invisible without a study that should not have been needed.
+            if (cfg.min_face_px > 0.f) {
+                const float side = std::min(best.bbox.width, best.bbox.height);
+                if (side < cfg.min_face_px) {
+                    std::cerr << "  [skip] face " << side << "px < " << cfg.min_face_px
+                              << "px: " << img_file.path().filename() << "\n";
+                    continue;
+                }
+            }
+
             cv::Mat crop = align_face(img, best.landmarks);
             if (crop.empty()) {
                 std::cerr << "  [skip] alignment failed: " << img_file.path().filename() << "\n";

src/gallery/gallery_builder.hpp

@@ -29,6 +29,16 @@ struct BuildConfig {
     float       detector_conf{0.5f};
     float       detector_nms{0.4f};
     int         max_side{500};  // downscale source images to this max dimension
+
+    /// Minimum detected-face side, in pixels of the (possibly downscaled)
+    /// source image. 0 disables the check.
+    ///
+    /// References below this are dropped rather than upscaled: a face smaller
+    /// than the embedder's input is off-distribution, and a bad reference
+    /// poisons every match against that identity for the life of the gallery.
+    /// Mirrors the inference-side --min-face-px so the gallery is built from
+    /// the same face scales it will be matched against.
+    float       min_face_px{0.f};
                                 // before detection — TMDB portraits are ~2k px,
                                 // SCRFD trains on smaller faces and detection
                                 // confidence drops on huge inputs. 0 = disabled.