feat(format): add runtime semantic-equality safety gate

pkg/lsp/server.go

@@ -124,6 +124,10 @@ func (s *server) handle(raw []byte) bool {
 			s.reply(req.ID, []textEdit{})
 			return false
 		}
+		if !format.SemanticallyEqual(text, formatted) {
+			s.reply(req.ID, []textEdit{})
+			return false
+		}
 		s.reply(req.ID, []textEdit{fullReplace(text, formatted)})
 	case "textDocument/rangeFormatting":
 		var p rangeFormattingParams
@@ -224,6 +228,9 @@ func (s *server) rangeFormat(text string, r lspRange) []textEdit {
 	if formatted == text {
 		return nil
 	}
+	if !format.SemanticallyEqual(text, formatted) {
+		return nil
+	}
 
 	// Split both versions into lines, keeping the trailing newline attached to
 	// each element so that joining them reconstructs the original string.