fix(frontmatter): consolidate 3 independent fence-parsers into one (#139)

Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/frontmatter/frontmatter.go

@@ -66,7 +66,7 @@ func Fix(doc []byte) ([]byte, bool, error) {
 		return doc, false, fmt.Errorf("frontmatter: quoting did not repair the block: %w", err)
 	}
 
-	_, rest, _ := split(doc)
+	_, rest, _ := Split(doc)
 	out := []byte(fence + "\n")
 	out = append(out, fixed...)
 	out = append(out, []byte(fence)...)
@@ -122,14 +122,25 @@ func needsQuoting(value string) bool {
 
 // block returns the raw YAML between the opening and closing fences.
 func block(doc []byte) ([]byte, bool) {
-	b, _, ok := split(doc)
+	b, _, ok := Split(doc)
 	return b, ok
 }
 
-// split returns the frontmatter block and everything from the closing fence
-// onward (the closing fence itself excluded, its trailing newline included).
-func split(doc []byte) (block, rest []byte, ok bool) {
-	s := string(doc)
+// Split returns doc's frontmatter block and everything from the closing
+// fence onward (the closing fence itself excluded, its trailing newline
+// included), or ok=false if doc has no properly-fenced frontmatter.
+//
+// The canonical fence detector for this codebase (hyperguild#138): three
+// independent reimplementations of this same logic each grew their own gap
+// through separate bug-fixing before being consolidated here —
+// internal/mcp's needed a CRLF/BOM guard added after both silently defeated
+// its own HasPrefix check, and internal/api's closing-fence match had no
+// trailing-newline requirement, so a document ending in "\n---extra" with no
+// real newline after the fence was treated as validly closed. Both classes
+// are closed here, once, for every caller.
+func Split(doc []byte) (block, rest []byte, ok bool) {
+	s := strings.TrimPrefix(string(doc), "\xef\xbb\xbf")
+	s = strings.ReplaceAll(s, "\r\n", "\n")
 	if !strings.HasPrefix(s, fence+"\n") {
 		return nil, nil, false
 	}
@@ -139,5 +150,18 @@ func split(doc []byte) (block, rest []byte, ok bool) {
 	if idx < 0 {
 		return nil, nil, false
 	}
-	return []byte(body[:idx+1]), []byte(body[idx+1+len(fence):]), true
+	// What follows the closing fence must be either a newline (a real body
+	// follows) or nothing at all (the fence is the literal end of the
+	// document — no body, not a bypass attempt). Anything else, e.g.
+	// "\n---extra", is not a valid close: an earlier version of this check
+	// accepted it, which is exactly the looser match hyperguild#138 closes.
+	after := idx + 1 + len(fence)
+	switch {
+	case after == len(body):
+		return []byte(body[:idx+1]), nil, true
+	case body[after] == '\n':
+		return []byte(body[:idx+1]), []byte(body[after:]), true
+	default:
+		return nil, nil, false
+	}
 }