A small markdown renderer for Issie's in-app help. Issie's longer help text - the Info window, the waveform simulator's help panels, the bodies of confirmation popups - used to be written as React element trees: `str` and `bSpan` and `li` interleaved with the words, several hundred lines of it. Read as prose it was unreadable, which is a problem for text whose whole job is to read well. It is now written as markdown in AppMessages and rendered here. The subset is not a guess: it is what that text was already doing, counted. Headings, bold, italic, inline code, links, bullet and numbered lists, and tables - which is core GitHub markdown with nothing left over. There are no images and no nested lists, so there is no support for them; a line that tries will render as its own text rather than silently vanish. TWO THINGS THIS DELIBERATELY DOES NOT DO. It does not produce an HTML string. Rendering markdown by handing HTML to `dangerouslySetInnerHTML` is the usual shortcut, and in a renderer with node integration, for messages that interpolate sheet names the user chose, it is a bad trade. `render` builds React elements, so there is no path from message text to executed markup. It does not open links itself. A markdown link becomes an anchor with an `onClick` supplied by the caller, because in Electron an ordinary `href` navigates the application window away from Issie. Keeping the handler out here also keeps this module free of Electron and of anything above it in compile order. `parse` is pure and returns data, so the tests can read every message in AppMessages under plain .NET without a browser. `render` is the only part that needs one.