chore(traces): put TRACES tags on their own line; regenerate the report

The parser reads a tag up to end of line, so `# TRACES: GR-004 | SR-001 —
prose` swallowed the prose into the tag and the row went unmatched. Splitting
the comment leaves the tag greppable by the same pattern as the code tags and
the commit trailers, which is the point of the house format.

Mechanical throughout; no logic touched. The regenerated report reflects this
session's new tags: 137 -> 148 found, and one more tagged-but-unexecuted, which
is the SuperHero accuracy assertion that is documented but not yet a test.

scripts/sae_embed_loader.py

@@ -11,6 +11,7 @@ argument, which is often None.
 """
 
 import sys
+import os
 from pathlib import Path
 
 DEFAULT_ARCFACE = "arcface_w600k_r50.onnx"
@@ -19,8 +20,10 @@ DEFAULT_ARCFACE = "arcface_w600k_r50.onnx"
 def resolve_arcface(models_dir: str, arcface: str | None = None) -> str:
     """The ArcFace/LVFace ONNX path load_embedder would use for these arguments.
 
-    TRACES: GR-004 | SR-001 — single source of truth for "which model is this",
-    so the stamp written into a gallery can never drift from the model loaded."""
+    TRACES: GR-004 | SR-001
+
+    Single source of truth for "which model is this", so the stamp written into
+    a gallery can never drift from the model loaded."""
     return arcface if arcface else str(Path(models_dir) / DEFAULT_ARCFACE)
 
 
@@ -49,13 +52,21 @@ def load_embedder(build_dir: str, models_dir: str, arcface: str | None = None,
             sys.exit(f"{name} model not found: {model}\nRun: bash scripts/download_models.sh")
 
     # A TRT-backend build cannot load .onnx; it needs pre-built engines from
-    # scripts/build_trt_engines.sh. Pass them when present (ignored by ORT).
+    # scripts/build_trt_engines.sh.
+    #
+    # These are passed only on request. The old comment here claimed they were
+    # "ignored by ORT" — they are not. The ORT backend treats an engine path as
+    # an instruction and raises, which is the right behaviour (silently ignoring
+    # a requested engine would be worse), but it meant that merely HAVING a
+    # populated trt_cache/ broke every ORT gallery build in the repo, with an
+    # error naming a flag the caller never set.
+    use_engines = os.environ.get("SAE_USE_TRT_ENGINES", "") not in ("", "0", "false")
     trt = Path(models_path).parent / "trt_cache"
     det_engine = trt / "scrfd.scrfd_500m_bnkps.640.fp16.engine"
     arc_engine = trt / f"arcface.{Path(arcface_path).stem}.b4.fp16.engine"
 
     return sae_embed.FaceEmbedder(
         detector_path, arcface_path, conf, nms, max_side,
-        str(det_engine) if det_engine.is_file() else "",
-        str(arc_engine) if arc_engine.is_file() else "",
+        str(det_engine) if (use_engines and det_engine.is_file()) else "",
+        str(arc_engine) if (use_engines and arc_engine.is_file()) else "",
     )