Drop the #6988 teardown-ordering caveat once holoviews ships the fix

[SimonHeybrock]

holoviz/holoviews#6988 was fixed upstream on 2026-08-19. The fix is not in a release yet: the latest is v1.23.1 (2026-07-02), and we pin holoviews==1.23.1.

The bug: _WeakSubscriber.__bool__ returns False for param-method subscribers, which defeats Plot.cleanup()'s owner filter, so cleanup severs every weakly-wrapped plot-refresh subscriber on the streams it touches, not just its own.

We do not have a workaround for this in the sense of extra code -- we have a constraint on the order of operations, plus comments explaining it. Once a release carries the fix and we bump the pin:

• Drop the ordering caveat from CellWidget.dispose's docstring in src/ess/livedata/dashboard/widgets/cell.py ("Any change to teardown must keep that order (sever first, render the replacement after) until #6988 is fixed"). Teardown of a displaced cell would no longer be able to sever a survivor's subscriptions, so the invariant stops being load-bearing.
• Revisit the same reasoning in Pop a cell's plot out into a floating window #1152 (pop-out windows), which has to argue the same all-or-nothing severing in plot_popout.py, plot_grid_tabs.py and .claude/rules/dashboard-widgets.md. Closing a pop-out unsubscribing the grid cell behind it is exactly the failure #6988 causes.

Explicitly not in scope: the plot.cleanup() block in CellWidget.dispose itself stays. That exists because of holoviz/panel#8710 (GridSpec never cleans up removed children), which is still open, and its TODO is already pointed there.