feat: enhance plugin system with Go helpers and add Modbus plugin

- Register Go helper functions (modbus.crc16, hex.encode/decode,
  util.bytes) into Lua states for Modbus RTU support
- Add plugins/modbus.lua with .modbus read/write commands
- Fix Reload race condition (hold lock across Unload+Load)
- Make App.Close nil-safe for sess
- Restore internal/console/console_test.go

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/app/app.go

@@ -182,7 +182,9 @@ func (a *App) Close() {
 		close(a.done)
 		a.forward.Close()
 		a.plugins.Close()
-		a.sess.Close()
+		if a.sess != nil {
+			a.sess.Close()
+		}
 		if a.logFile != nil {
 			_ = a.logFile.Close()
 		}