Interface: SheetBaseStatics

The statics a VTTForge sheet base carries.

The factory used to return a bare constructor, so a subclass writing super.DEFAULT_OPTIONS (the pattern the docs show and every sheet needs) failed to compile. TypeScript cannot see a static through an untyped constructor. The example system never caught it because it is JavaScript.

DEFAULT_OPTIONS is deliberately loose: a subclass merges its own shape into it, and pinning ours would reject the merge.