[Core Port Req.] Various Atari Emulators. (Based on the systems from 1977 and 1992)

Right, this is the big one I'm going to list out three two potential emulators, they all share the same Atari Systems and listing it out mainly for which is considered as the "non-spaghetti code".
Also cheating by using this section from TASVideos that's convenient in the potential ones.

Altirra

• Website
• Target platforms: Win7+ (and Linux via WINE)
• Source distributed as archives
• Language(s): C / C++
• License: GPLv2
• Systems: Atari 400/800, 1200XL, 600/800XL, 130XE, XEGS, and 5200

Atari800

• Website
• Target platforms: Android, Linux, Win7+, several others
• Source repo
• Language(s): C
• License: GPLv2
• Systems: Atari 400/800, 600XL/800XL, 130XE and Atari 5200

Other emulators

• Atari++: available only as source archives; C++; choice of (butchered MPL) or (GPL, unspecified version)
• kat5200: source distributed as archives; C/C++; GPLv1+

Merits

Honestly it would be nice to have that middle gap between the 2600 and the 7800 be filled out by getting 5200 in the emulator, and these two do that and more.

Start by reviewing the Atari800 and Altirra sources linked in the issue, then compare the listed Atari systems and licensing details with BizHawk's existing core-port requirements. Done would mean selecting and integrating an emulator for the requested Atari systems, but the issue does not name BizHawk files, tests, or a specific implementation scope.