feat(active): report a run of images whose order carries a value

Session 6 of the 2026 threat plan (attack row 3): A05 output was red.
CamoLeak renders a secret as one image per character from a URL alphabet;
each URL is ordinary, so each image graded LOW and the gate WARNed.

- active:markdown-image-sequence (MEDIUM): at least 4 ordinary external
  images, inline or reference-style, on one URL template whose varying
  tokens are 2 chars or fewer, or digit runs. Digits that count up in
  document order are a counter and carry nothing. The per-image grade is
  unchanged; the finding sits beside it.
- Accepted twins (flag tables, figures 1a/1b) go to review; measured
  evasions (word dictionary, 3-char symbols, counter increments, run of 3,
  a template per symbol) are in LIMITATIONS.

Guard rule before/after: FP 106/198, 196/2918, 108/389 identical. Those
corpora hold no external markdown image, so the rule was also run on 577
package READMEs (312 with external images): 0 firings at run lengths 3-8.
OKF import 47/31 identical. Matrix 140/140, ReDoS 163 patterns 0 flagged,
suite 1179 passed + 5 xfailed. Row 3: 13 caught, 1 gap, 5 open.

tests/test_output.py

@@ -428,6 +428,9 @@ _REDOS_PAYLOADS = [
     ("active-md-image-url", scan_active_content, "![a]("),
     ("active-md-link-url", scan_active_content, "[a]("),
     ("active-html-tag-attrs", scan_active_content, "<a"),
+    # 1.7.0: the reference-style image pattern behind the sequence rule. Its alt
+    # run is the image row's `![` above; the label run behind it needs its own.
+    ("active-md-image-ref-label", scan_active_content, "![a]["),
     # 0.6.0 put a value parser behind the URL-attribute presence test. Its one run
     # is the `\s*` in front of the required `=`, so the unit has to DENY the `=`:
     # a unit that supplies it matches immediately and never exercises the run.