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/build_gallery.cpp

@@ -34,6 +34,7 @@ int main(int argc, char** argv) {
     std::string arcface_model  = kDefaultArcfaceModel;
     float conf = 0.5f, nms_thr = 0.4f;
     int   max_side = 500;
+    float min_face_px = 0.f;
 
     for (int i = 1; i < argc; ++i) {
         auto arg  = [&](const char* f) { return std::strcmp(argv[i], f) == 0; };
@@ -50,6 +51,7 @@ int main(int argc, char** argv) {
             else if (arg("--conf"))      conf           = std::stof(next());
             else if (arg("--nms"))       nms_thr        = std::stof(next());
             else if (arg("--max-side"))  max_side       = std::stoi(next());
+            else if (arg("--min-face-px")) min_face_px = std::stof(next());
             else { std::cerr << "[warn] unknown flag: " << argv[i] << "\n"; }
         } catch (const std::exception& e) {
             std::cerr << "Error: " << e.what() << "\n";
@@ -59,7 +61,8 @@ int main(int argc, char** argv) {
 
     if (root_path.empty() || output_path.empty()) {
         std::cerr << "Usage: build_gallery --root <dir> --output <gallery.json> "
-                     "[--detector <path>] [--arcface <path>] [--max-side <N>]\n";
+                     "[--detector <path>] [--arcface <path>] [--max-side <N>]\n"
+                     "       [--min-face-px <px>]\n";
         return 1;
     }
 
@@ -70,6 +73,7 @@ int main(int argc, char** argv) {
     cfg.detector_conf  = conf;
     cfg.detector_nms   = nms_thr;
     cfg.max_side       = max_side;
+    cfg.min_face_px    = min_face_px;
 
     try {
         ActorGallery gallery = build_gallery(cfg);