Register the Doubao video mark with a corpus-calibrated single-floor policy

detect_doubao_frame reuses the image engine's synthetic doubao alpha as its
video template, so the glyph cannot drift between registries. The policy is
seedance-style single-floor, calibrated by sequential decode over 39
TC260-confirmed Doubao corpus videos and 25 negatives: a 12-frame stable run
at 0.35 accepts 33/39 positives and 0/25 negatives (0.30 already accepted a
negative), and through the shipped arbiter identify_video selects 32/39 with
no false selection. Also documents the measured impossibility of a bare-pill
removal arm with the current detector (68 OCR-confirmed pills max 0.317 vs
clean negatives max 0.353 -- no separating threshold exists).

scripts/render_visible_examples.py

@@ -187,6 +187,8 @@ def _video_mark_frame(key: str, w: int, h: int) -> np.ndarray:
         return _composite_light(base, cv2.resize(tmpl, (tw, th)).astype(np.float32) / 255.0, x, y, 250)
     elif key == "seedance":
         tmpl, scale, x, y = templates["seedance"], 0.095, int(w * 0.74), int(h * 0.80)
+    elif key == "doubao":
+        tmpl, scale, x, y = templates["doubao"], 0.048, int(w * 0.72), int(h * 0.82)
     elif key == "dola":
         tmpl, scale, x, y = templates["dola"], 0.036, int(w * 0.70), int(h * 0.88)
     elif key == "hailuo":