Help add-in documentation page and screenshots, settings and references-only packages, and a VFS root Parent crash

[KubaO]

Adds the help add-in's documentation page and the tool that takes its pictures, narrows the add-in to the packages a project references, and files a compiler crash found while probing.

• docs: Help (new; 927 to 928 pages) under IDE add-ins covers F1 and the pane, hover help, search, several pages, the project's own procedures, settings, the help window and building it. It has eight screenshots. API-Declarations gains native callers of a ThisCall implementation, the Sub + Err.ReturnHResult route past [PreserveSig], and how the type library records each convention (checked on BETA 997 with MSVC).
• help add-in:
  • A gear in the pane's bar and the window's title bar opens a settings panel holding Hover help.
  • The add-in offers only the packages the project references, read from the VFS Packages folder. A new All packages setting offers every one.
• shoot_help_addin (new): writes the page's screenshots from an IDE on a private desktop (dark, 2x). A picture is rewritten only when its bytes differ, and the run fails if the Windows user name shows in the page. test/addin/helpdemo is the project it operates.
• bugs: reading Parent of the VFS root crashes the compiler. It has a BUGS entry with a reproducer and a WARNING on FileSystemItem.
• tests: the new addin-test.bat lanes are references (probe P20) and parent (P21). The help lane covers the settings and the package list.

shoot_help_addin and the new lanes need a twinBASIC install, so they stay outside every gate and CI. Their command-line cases need none and run in test.bat.