feat: show specific toast on top (priority per toast call)

by default the last toast call shows up as the last one.
i need the ability to show a certain toast on top of the existing ones when i need to.

No files, tests, or entry points are named. Start by reproducing the current toast ordering and tracing the toast-call API; done should mean a caller can select a toast priority so that toast appears above existing ones, with tests covering the ordering.

Written by the indexing model from the issue text.