freshly downloaded file is damaged

I am sorry to hear about this. As a first troubleshooting step, you will need to ensure that your local account on the MacOS device has full administrator privilege to read/write/access your hard drive. You can find more information on administrator privileges for local mac user accounts here: https://support.apple.com/en-ca/guide/mac-help/mtusr001/mac
 
If you have tried the above and the issue persists, you should be able to do a workaround that will allow you to play the game on your Mac OS in the meantime:
 
1) On your Steam Library list, if you click on PLAY for Don't Starve and you will encounter the error message indicating the file is damaged and should be moved to trash, click on the cancel button to close that window.

2) Instead, go back to your Steam Library list, right-click on Don't Starve > Click on "Manage" > and click on "Browse Local Files"; this will open up your installation folder.

3) Right-click on the donotstarve_steam app > select the OPEN button; this will provide another error message indicating the following:
"dontstarve_steam is damaged and can't be opened. You should move it to the trash."

4) On that error message, click on the OPEN button. This will force the Mac OS to launch the game successfully. Once you are able to successfully launch it once this way, you only need to click on the PLAY button on Steam to launch it going forward.