Add a comprehensive step-by-step guide for injecting a custom main-menu button from an F4SE C++ plugin (docs/MainMenu_Button_Injection_Guide.md). Update existing docs to reference the new guide: add link in docs/MainMenu_Injection.md and expand the setup note in docs/setup.md to point to the full how-to and project-specific notes.
Introduce a C++ runtime injection that adds a Multiplayer row to the Fallout 4 main menu instead of patching MainMenu.swf. Adds F4TMainMenuInject.h/cpp implementing injection, ITEM_PRESS handling, accept/button hooks, resync tasks and logging; integrates installation by calling MainMenuInject::Install() from PrismaUI init. Update PrismaUI to use the new helper and remove a duplicate IsMainMenuOnMainPanel implementation. Documentation updated to deprecate FFDec SWF patching (new docs/MainMenu_Injection.md, note in FFDEC guide, and setup.md) and restore-vanilla-mainmenu.ps1 messaging updated to instruct using the runtime injection.
andrew