Register the Qwen 千问AI生成 visible text mark

Calibrated on the 117-frame TC260-producer cohort (vendor_cohort_harvest +
vendor_mark_calibrate, both committed here): per-mark 2-rung ladder
(0.78, 1.27) for the two measured size modes, fitted locate box (the mark
sits ~0.025 of the short side off the edge; doubao's box clipped the first
glyph), measured template aspect 0.26, gate 0.45 (clean p99 0.301).
Strict-only (the sub-gate band is non-Qwen banners), no rival margin
(0 cross-fires on 400 doubao / 298 jimeng / 286 clean frames).
83/83 real marks detector-clean after cv2 fill.

TextMarkConfig gains a per-mark ladder field; the shipped 3-rung default
is unchanged for every other mark.

scripts/real_examples_e2e.py

@@ -180,7 +180,7 @@ def check_visible(res: Results, tmp: Path) -> None:
     from remove_ai_watermarks.watermark_registry import detect_marks, get_mark, remove_auto_marks
 
     print("\nvisible --mark auto -- real marked image per mark, product decision then re-detect")
-    for mark in ("doubao", "jimeng", "gemini", "samsung", "jimeng_pill"):
+    for mark in ("doubao", "jimeng", "qwen", "gemini", "samsung", "jimeng_pill"):
         src = find_visible_positive(mark)
         if src is None:
             res.add("visible", mark, True, "no live positive in bucket (skipped, not a failure)")