v0.3.1: a clear message instead of a crash when Portal 2's data is incomplete

Fix for the startup crash reported on Reddit (host SIGBUS on guest address 0x7c). Portal 2 loads the puzzle-editor item definitions even for the campaign; when portal2_dlc2/scripts/editoritems.txt is missing or empty, the game dereferences a null item and crashes. Now:

• the installer and setup.sh check that file before building, and prefer a complete depot_621 folder over an incomplete Steam install;
• the app checks the file it will really read (user overrides included) before starting Steam, and shows a message naming the folder and the fix instead of crashing.

If you see "Portal 2 game data is incomplete": an empty placeholder file does not help, the game needs the real one. In Steam, right-click Portal 2 > Properties > Installed Files > Verify integrity of game files; or download the complete data with download_depot 620 621 in the Steam console and choose depot_621 in the installer. No rebuild is needed: the data is read at launch.

Also: the Half-Life 2 download guidance and depot detection were fixed.

Opening the installer: signed with a Developer ID and notarized by Apple, so it opens like any downloaded app. Unzip it and open Natif Installer. Or git clone the repository and run ./setup.sh.

You must own the games: everything is built from your own copy, and nothing from Valve is in the repository. Unofficial fan project, not affiliated with or endorsed by Valve.

SHA-256 of the zip: ed0f09dfc5b9c120371906264b54f5e00056f9fbe3d8097be3a0024dd2fed789