Fix headless callbacks

[AdamZ-8113]

Description of the problem being solved:

This fixes the mainObject nil error during headless startup. HeadlessWrapper.lua still had old callback functions that overrode the ones loaded from _SimpleGraphic.def.lua. Those functions were setting the old local variables, while the newer wrapper was reading mainObject and callbackTable.

Steps taken to verify a working solution:

Tested locally against tests-branch at 59c25d23 using the official PoB test container.

• Confirmed the startup error before the fix and successful startup afterward.
• Loaded and calculated all 500 builds from the live pob.codes API in batches of 50. Also checked callback registration and dispatch.
• Ran the existing unit tests: 583 passed, with one failure in the trade-query 401 response test.

Link to a build that showcases this PR:

Before screenshot:

After screenshot: