Function has_main_poster 

pub fn has_main_poster() -> bool

Whether a backend has installed the main poster yet — i.e. whether on_main would work rather than panic. Platform glue that can be called BEFORE launch (a notification tap that cold-starts the process) probes this and buffers instead of posting. The poster is set once and never cleared, so a false can only become true: a caller that buffers on a false negative is still correct, provided something drains the buffer at launch.