fix(spec): don't panic on invalid UTF-8 in requirement names

applyRename built a regexp with regexp.MustCompile from an unescaped
requirement name; a name containing invalid UTF-8 (or a bad pattern) panicked
the whole process. Use regexp.Compile and fall back to leaving the content
unchanged, and ReplaceAllLiteralString so $$ in the new name is not treated as
a group reference. Found by the new FuzzParseDeltaSpec target.

Author: Patel230

internal/spec/delta_fuzz_test.go

@@ -11,6 +11,9 @@ func FuzzParseDeltaSpec(f *testing.F) {
 	f.Add("## ADDED Requirements\n")
 	f.Add("## REMOVED Requirements\n### Requirement: x\n**Reason**: y")
 	f.Add("## RENAMED Requirements\n### Requirement: x\n- FROM: a\n- TO: b")
+	// Regression: a RENAMED requirement name with invalid UTF-8 used to panic
+	// in applyRename's regexp.MustCompile.
+	f.Add("## RENAMED Requirements\n### Requirement: x\n- FROM: \xff0\n- TO: y")
 
 	f.Fuzz(func(t *testing.T, content string) {
 		ds, err := ParseDeltaSpec(content)

internal/spec/delta_merge.go

@@ -72,8 +72,14 @@ func deleteRemoved(content string, req DeltaRequirement) string {
 func applyRename(content string, req DeltaRequirement) string {
 	oldName := regexp.QuoteMeta(req.OldName)
 	newName := req.NewName
-	re := regexp.MustCompile(`(?m)^### Requirement: ` + oldName + `$`)
-	return re.ReplaceAllString(content, "### Requirement: "+newName)
+	re, err := regexp.Compile(`(?m)^### Requirement: ` + oldName + `$`)
+	if err != nil {
+		// An unparseable name (e.g. invalid UTF-8) must not panic; leave the
+		// content unchanged.
+		return content
+	}
+	// Literal replacement so "$" in the new name is not treated as a group ref.
+	return re.ReplaceAllLiteralString(content, "### Requirement: "+newName)
 }
 
 // renderRequirementBlock renders a delta requirement as markdown.