ENH: Implement TimeChange behaviour for evoked_topo and recursive option in link

[Gnefil]

Reference issue (if any)

Implements #13928.

What does this implement/fix?

It enables plot_evoked_topo figures to interact with other figures sharing TimeChange event.

The strategy is to implement on_time_change function, link subfigures with the main figure, and mark the current time (set as a figure attribute) with a vertical line. Clicks on the subfigures publish TimeChange events, main figure subscribes to external events, and publish/subscribe transmit events/callbacks through the link function.

Additional information

ui_events.link() function now has a merge parameter that allows mutually linking the link-groups that figs belong to separately.

[Copilot]

Pull request overview

This PR adds TimeChange-based UI event interoperability to mne.viz.plot_evoked_topo (enabling time synchronization with other interactive figures) and extends mne.viz.ui_events.link() with a merge option to merge existing link-groups.

Changes:

• Add TimeChange publishing/subscribing to the magnified (single-channel) topo time series and track current time on the main plot_evoked_topo figure.
• Extend mne.viz.ui_events.link() with merge= to unify existing link-groups.
• Add/adjust tests, a tutorial snippet, and a changelog entry covering the new behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tutorials/evoked/20_visualize_evoked.py	Adds an example linking an interactive topomap and plot_evoked_topo for synchronized time selection.
mne/viz/ui_events.py	Adds merge parameter to link() to merge figures’ existing link-groups.
mne/viz/topo.py	Implements TimeChange interactions for the magnified topo time series and subscribes the main topo figure to time-change events.
mne/viz/tests/test_ui_events.py	Adds a test covering merge=True link-group behavior.
mne/viz/tests/test_topo.py	Adds tests for time-change propagation/select-line behavior in plot_evoked_topo.
doc/changes/dev/13968.newfeature.rst	Adds a changelog entry describing the new TimeChange behavior and merge parameter.

[wmvanvliet]

Can you also document the UI events that plot_evoked_joint now publishes/subscribes to?
Does plot_evoked_topo also changle the TimeChange event?

    Notes
    -----
    The figure will publish and subscribe to the following UI events:

    * :class:`~mne.viz.ui_events.TimeChange`

[Gnefil]

Updated according to second round of reviews.

[wmvanvliet]

Code looks good. Just two minor things