The persisted view stack was restored verbatim. RESTORABLE_VIEWS stopped the
popup opening ONTO a view it will not re-render, but nothing kept such a view
out of the stack, so Back could land on a screen whose content was deliberately
never restored. No secret leaks -- those views are blank precisely because
nothing is restored into them; this is a navigation defect.
loadState() now truncates the stored stack at the first entry outside
RESTORABLE_VIEWS, dropping it and everything above it. Truncating rather than
splicing keeps the result a prefix of what was stored, so every surviving entry
keeps the Back target it had; splicing would silently re-point the entry above
the hole at a different screen. Filtering on load rather than on save is what
makes it retroactive for stacks already in storage, and leaves the live
in-session stack whole, which it should be.
The general case where Back lands on a blank screen even for restorable views,
because goBack() re-renders nothing, is separate and tracked at #268.
