I'm on board for these changes, although removing Ord makes sense if we don't have a reasonable use case for it, especially given the conversation above.

I think I'm even on board for WindowRef::Primary != WindowRef::Entity(PRIMARY_WINDOW_ENTITY), provided we document that.

I kinda think that's a problem with the new RenderTarget changes. I've found the recent changes to the type difficult to work with, and the special casing for primary windows makes it awkward to use. The fact that you can represent identical states with different values is a code smell to me, and not something I think we should patch over with more code.

The core issue is that you can't have a statically defined "default entity id" (like we could for WindowId::primary because we owned the entire id space for WindowId). If we remove WindowRef::Primary, that would mean that RenderTarget could no longer have a default window value, which means Cameras could no longer have a default value.

There are a variety of alternatives, I just think they're all worse than the current approach:

1. Re-add WindowId and make it a component. For RenderTargets and other window references, use that instead of Entities. This feels weird to me (each window has two canonical ids) and makes a number of assignment scenarios harder than entities (ex: spawn a window with a command, grab the newly created WindowId, assign to camera render target).
2. Use RenderTarget::Window(Option<Entity>) and "fix up" the None value on the first frame with the primary window. I've been pushing against the "fix up" pattern in Bevy for awhile. As much as possible I don't want to be touching people's entities. If a user sets a value on a component, it should stay that way to avoid surprises. This pattern also produces change events that users don't expect. And imo as much as possible entities should be in a "correct" state on insert. They shouldn't be "made correct' at some arbitrary point in the schedule. This also doesn't really solve the comparison problem because None != Some(PRIMARY_WINDOW_ENTITY) is essentially the same thing.