review: restore astutil.CountLeadingSpaces in isPIOpenLine for consistency

Round-3 review found that the round-2 efficiency fix (inline
len(raw)-len(trimmed) > 3) created inconsistency with the identical
CommonMark "≤3 leading spaces" gate in sibling functions fenceOpenRun
and fenceClose, both of which call astutil.CountLeadingSpaces. Restore
the named-helper call so all three functions express the same spec rule
the same way; the second TrimLeft on this path is negligible.

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

Author: claude

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 {
-	trimmed := bytes.TrimLeft(raw, " ")
-	if len(raw)-len(trimmed) > 3 {
+	if astutil.CountLeadingSpaces(raw) > 3 {
 		return false
 	}
+	trimmed := bytes.TrimLeft(raw, " ")
 	trimmed = bytes.TrimRight(trimmed, " \t\r\n")
 	if !bytes.HasPrefix(trimmed, piOpenPrefix) {
 		return false