NestTabs.SaveAs writes the tab's nest, makes the file its file and name,
and clears its unsaved mark; a write that fails throws and changes
nothing. ExportCopy writes the nest without touching its file, name or
unsaved mark.
Both refuse, before writing anything, a file another open tab shows (so
one file never has two tabs) and a tab that has been closed; Export
also refuses the tab's own file, which only Save may write. Refusals
are a SaveTargetRefusedException whose message says what to do.
SavePath is the file Save writes without asking: only one saved to this
session, as in the WinForms app, so a nest that was only opened asks
for a file on its first Save.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
