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.

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"]