Right-click any tab and pick Reload: the pane's content remounts in
place — effects re-run, state resets, measurements are retaken — while
the tab keeps its slot and every other tab is untouched.
A per-pane epoch atom keys the contribution inside the zone body, so
reload never rewrites the layout tree. Both tab menus offer it: the
zone strip menu (tool panels, the file tree, a fresh draft's main tab)
and the session tab menu (tiles + the loaded main tab).
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