astutil: walk inline descendants in HeadingLine; add missing tests

HeadingLine previously only checked direct children for *ast.Text,
so ATX headings with an emphasis or link as first child (e.g. ## *foo*
or ## [t](u)) on a later line fell back to 1 instead of the correct
line.  Switch to ast.Walk over all inline descendants, stopping at the
first *ast.Text, matching the headingstyle approach.

Add TestHeadingLine_ATXEmphasisOnLaterLine and
TestHeadingLine_ATXLinkOnLaterLine to cover this path.

https://claude.ai/code/session_01YbFAKES95Szi7i251Yv1XE

Author: claude

internal/rules/astutil/astutil.go

@@ -9,18 +9,28 @@ import (
 
 // HeadingLine returns the 1-based source line of a heading node.
 // Setext headings expose their line via Lines(); ATX headings are found
-// by walking child text nodes.  Returns 1 as a safe fallback.
+// by walking inline descendants until the first text segment. Returns 1
+// as a safe fallback.
 func HeadingLine(heading *ast.Heading, f *lint.File) int {
 	lines := heading.Lines()
 	if lines.Len() > 0 {
 		return f.LineOfOffset(lines.At(0).Start)
 	}
-	for c := heading.FirstChild(); c != nil; c = c.NextSibling() {
-		if t, ok := c.(*ast.Text); ok {
-			return f.LineOfOffset(t.Segment.Start)
+
+	line := 1
+	_ = ast.Walk(heading, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering || n == heading {
+			return ast.WalkContinue, nil
 		}
-	}
-	return 1
+		t, ok := n.(*ast.Text)
+		if !ok {
+			return ast.WalkContinue, nil
+		}
+		line = f.LineOfOffset(t.Segment.Start)
+		return ast.WalkStop, nil
+	})
+
+	return line
 }
 
 // ParagraphLine returns the 1-based source line of a paragraph node.

internal/rules/astutil/astutil_test.go

@@ -69,6 +69,47 @@ func TestHeadingLine_ATXOnLaterLine(t *testing.T) {
 	assert.Equal(t, 3, line)
 }
 
+func TestHeadingLine_ATXEmphasisOnLaterLine(t *testing.T) {
+	// ATX heading on line 3 whose only child is emphasis (not a direct *ast.Text).
+	// HeadingLine must descend into inline children to find the text segment.
+	src := []byte("Text\n\n## *emph*\n")
+	f, err := lint.NewFile("test.md", src)
+	require.NoError(t, err)
+
+	var line int
+	_ = ast.Walk(f.AST, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+		if h, ok := n.(*ast.Heading); ok {
+			line = HeadingLine(h, f)
+			return ast.WalkStop, nil
+		}
+		return ast.WalkContinue, nil
+	})
+	assert.Equal(t, 3, line)
+}
+
+func TestHeadingLine_ATXLinkOnLaterLine(t *testing.T) {
+	// ATX heading on line 3 whose only child is a link node.
+	src := []byte("Text\n\n## [link](url)\n")
+	f, err := lint.NewFile("test.md", src)
+	require.NoError(t, err)
+
+	var line int
+	_ = ast.Walk(f.AST, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+		if h, ok := n.(*ast.Heading); ok {
+			line = HeadingLine(h, f)
+			return ast.WalkStop, nil
+		}
+		return ast.WalkContinue, nil
+	})
+	assert.Equal(t, 3, line)
+}
+
 func TestHeadingLine_Fallback_Returns1(t *testing.T) {
 	heading := ast.NewHeading(1)
 	f, err := lint.NewFile("test.md", []byte("# X\n"))