review: fix double-scan, allocation regression, and test gaps

Four cleanup fixes from the round-2 code review pass:

- requiredstructure/isPIOpenLine: eliminate redundant double-scan of
  the leading-space prefix (CountLeadingSpaces then TrimLeft on same
  raw). One TrimLeft now derives both the trimmed slice and the indent
  count via len difference.

- orderedlistnumbering/checkListLines,checkItem: revert strconv.Itoa +
  string concatenation back to fmt.Sprintf. The concat form makes 3-4
  allocations per diagnostic versus Sprintf's ~1 via its pooled pp.

- astutil_test/TestCollectSectionHeadings_Memoized: add require.Len
  guard on h2 so a broken memoization path produces a clear assertion
  failure instead of a panic on h2[0].

- astutil_test/TestCountLeadingSpaces: add the space-then-tab boundary
  case (" \tabc" → 1) to the canonical unit test; the case existed in
  listindent/helpers_test.go but not in astutil's own test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gj8AvSb7r12iqa7r3vYYof

Author: claude

internal/rules/astutil/astutil_test.go

@@ -534,6 +534,7 @@ func TestCollectSectionHeadings_Memoized(t *testing.T) {
 	h1 := CollectSectionHeadings(f)
 	h2 := CollectSectionHeadings(f)
 	require.Len(t, h1, 3)
+	require.Len(t, h2, 3)
 	assert.Same(t, &h1[0], &h2[0],
 		"repeated calls must return the cached slice")
 }
@@ -821,6 +822,7 @@ func TestCountLeadingSpaces(t *testing.T) {
 	assert.Equal(t, 2, CountLeadingSpaces([]byte("  abc")))
 	assert.Equal(t, 0, CountLeadingSpaces([]byte("")))
 	assert.Equal(t, 3, CountLeadingSpaces([]byte("   ")))
+	assert.Equal(t, 1, CountLeadingSpaces([]byte(" \tabc")))
 }
 
 // --- IsBlank ---

internal/rules/orderedlistnumbering/rule.go

@@ -135,8 +135,7 @@ func (r *Rule) checkListLines(f *lint.File, listStart int, itemLines []int) []li
 		}
 		if i == 0 && startMismatch {
 			diags = append(diags, r.diag(f, line,
-				"ordered list starts at "+strconv.Itoa(listStart)+
-					"; configured start is "+strconv.Itoa(r.Start)))
+				fmt.Sprintf("ordered list starts at %d; configured start is %d", listStart, r.Start)))
 		}
 		if startMismatch {
 			continue
@@ -163,8 +162,7 @@ func (r *Rule) checkItem(f *lint.File, line, i int) (lint.Diagnostic, bool) {
 		return lint.Diagnostic{}, false
 	}
 	return r.diag(f, line,
-		"ordered list item "+strconv.Itoa(i+1)+" numbered "+strconv.Itoa(literal)+
-			"; expected "+strconv.Itoa(expected)), true
+		fmt.Sprintf("ordered list item %d numbered %d; expected %d", i+1, literal, expected)), true
 }
 
 func (r *Rule) diag(f *lint.File, line int, msg string) lint.Diagnostic {

internal/rules/requiredstructure/rule.go

@@ -1475,10 +1475,10 @@ func fenceRun(line []byte, ch byte) int {
 // (the bytes up to the first whitespace or `?>`). An indented code
 // example showing a directive is therefore not mistaken for one.
 func isPIOpenLine(raw []byte) bool {
-	if astutil.CountLeadingSpaces(raw) > 3 {
+	trimmed := bytes.TrimLeft(raw, " ")
+	if len(raw)-len(trimmed) > 3 {
 		return false
 	}
-	trimmed := bytes.TrimLeft(raw, " ")
 	trimmed = bytes.TrimRight(trimmed, " \t\r\n")
 	if !bytes.HasPrefix(trimmed, piOpenPrefix) {
 		return false