Sever pipe subscriptions when a cell widget is disposed

[SimonHeybrock]

Fixes #1224.

A cell rebuilt while its grid is visible left the discarded widget's rendered plot subscribed to the layers' hv.streams.Pipe: replacing the widget in the GridSpec slot does not run Panel's pane cleanup, so every SessionLayer.update_pipe drove the dead plot alongside the live one for the rest of the session — roughly doubling the dominant cost of the poll pass per rebuild, monotonically (see the issue for measurements).

CellWidget now keeps a handle on the pn.pane.HoloViews it builds, and dispose() runs Plot.cleanup() on the rendered plots and drops the pane's plot registry. dispose() already runs on rebuild, orphan removal, and document teardown, so all paths are covered.

One subtlety discovered while testing: Plot.cleanup()'s owner filter is defeated by holoviews' _WeakSubscriber wrapper (its __bool__ is False for any param-method subscriber, current upstream included), so cleanup severs every weakly-wrapped plot-refresh subscriber on the touched streams, not only its own. This is safe here because a layer's pipe is per session and per cell, and a rebuild disposes the displaced widget before the replacement renders — the docstring records the reasoning. Filed upstream as holoviz/holoviews#6988; the missing GridSpec cleanup itself is holoviz/panel#8710 (in-place objects mutation defeats the removed-child diff).

The sever uses the pane's private _plots registry, mirroring the pipe half of pn.pane.HoloViews._cleanup: Panel only runs that from a document root we no longer have, and exposes no public teardown.

Test plan

• Regression tests: dispose unsubscribes a rendered plot; a rebuild-while-rendered keeps the pipe-subscriber count flat. Both fail without the fix.
• Manual check with two sessions (one forcing rebuilds of the other's visible grid) per the measurement recipe in Cell rebuilt while its grid is visible leaves the old plot subscribed to the layer pipe #1224:
  • subscriber count stays at steady state across rebuilds

🤖 Generated with Claude Code

[SimonHeybrock]

LGTM