fix(visual_report): make TOC heading slugs unique

Ensure generated visual-report TOC slugs cannot collide with naturally occurring slug names. Add regression coverage for duplicate headings, natural suffix collisions, and unchanged distinct headings.

src/visual_report.py

@@ -118,15 +118,23 @@ def _extract_headings(md_text: str) -> List[Dict[str, str]]:
         return re.sub(r'\s+', ' ', text).strip()
 
     def _make_slug(text: str) -> str:
-        slug = re.sub(r'[^a-z0-9]+', '-', text.lower()).strip('-')
-        if not slug:
-            slug = "section"
-        if slug in seen_slugs:
-            seen_slugs[slug] += 1
-            slug = f"{slug}-{seen_slugs[slug]}"
-        else:
-            seen_slugs[slug] = 0
-        return slug
+        base = re.sub(r'[^a-z0-9]+', '-', text.lower()).strip('-')
+        if not base:
+            base = "section"
+        if base in seen_slugs:
+            # Increment until the disambiguated candidate is itself unused, so a
+            # generated "intro-1" can't collide with a natural "intro-1" slug.
+            n = seen_slugs[base]
+            while True:
+                n += 1
+                cand = f"{base}-{n}"
+                if cand not in seen_slugs:
+                    break
+            seen_slugs[base] = n
+            seen_slugs[cand] = 0
+            return cand
+        seen_slugs[base] = 0
+        return base
 
     for m in re.finditer(r'^(#{2,3})\s+(.+)$', md_text, re.MULTILINE):
         level = len(m.group(1))

tests/test_visual_report_slug_unique.py

@@ -0,0 +1,27 @@
+"""Regression: _extract_headings must emit a unique slug per heading.
+
+_make_slug disambiguates repeats by appending "-N", but it only tracked the
+*base* slug, so a generated "intro-1" could collide with a naturally-occurring
+"intro-1" (e.g. headings "Intro", "Intro", "Intro 1" all produced
+["intro", "intro-1", "intro-1"]). Duplicate slugs become duplicate heading ids,
+which makes the second table-of-contents link dead. Slugs are now guaranteed
+unique. Plain repeats keep their existing "-1", "-2" sequence.
+"""
+from src.visual_report import _extract_headings
+
+
+def _slugs(md):
+    return [h["slug"] for h in _extract_headings(md)]
+
+
+def test_disambiguated_slug_does_not_collide_with_natural_slug():
+    slugs = _slugs("## Intro\n\n## Intro\n\n## Intro 1\n")
+    assert len(slugs) == len(set(slugs)), slugs
+
+
+def test_plain_repeats_keep_sequential_suffixes():
+    assert _slugs("## Foo\n\n## Foo\n\n## Foo\n") == ["foo", "foo-1", "foo-2"]
+
+
+def test_distinct_headings_are_unchanged():
+    assert _slugs("## Alpha\n\n## Beta\n") == ["alpha", "beta"]