implement interpreter and demo + bug fixes

- implement interpreter and demo
- replace StandardError with appropriate subclasses
- implement division
- rewrite Polynomial.simplify()

bin/demo

@@ -4,21 +4,27 @@
 require 'bundler/setup'
 require 'ruby_algebra'
 
-puts 'RubyMaple © RubyMaple Creators, 2026. No rights reserved.'
+print "RubyMaple © RubyMaple Creators, 2026. No rights reserved.\nPress Ctrl+D to quit.\n"
 
+interp = RubyAlgebra::Interpreter.new
 loop do
-  puts 'Введите формулу (пустая строка для выхода): '
-  inp = $stdin.gets.chomp
-  exit if inp.empty?
-  begin
-    formula = RubyAlgebra::Parser.parse(inp)
-  rescue StandardError
-    puts 'Ошибка синтаксиса'
-  end
-  puts "Формула: #{formula}"
-  puts "Производная по x: #{formula.diff('x')}"
-  puts "Производная по y: #{formula.diff('y')}"
+  print '> '
+  inp = $stdin.gets
+  break if inp.nil?
 
-  value = formula.evaluate
-  puts "Числовое значение: #{value}" unless value.nil?
+  inp = inp.chomp
+  next if inp.empty?
+
+  begin
+    command = RubyAlgebra::Parser.parse_command(inp)
+  rescue StandardError => e
+    puts "Syntax error: #{e.message}"
+    next
+  end
+  begin
+    puts "< #{interp.execute(command)}"
+  rescue StandardError => e
+    puts "Calculation error: #{e.message}"
+  end
 end
+puts