Add an example recluster plugin for the Template GUI (ISO-SPLIT / Gaussian mixture)

[Sachuriga]

Summary

The Template GUI has no "recluster" action — the Kwik GUI's shift+ctrl+K, which
re-runs KlustaKwik2 on a selected cluster, has no counterpart. This adds one as an
example plugin: it re-clusters a selected cluster from the PC features that
already exist in the sorting output.

No spike detection is re-run, no raw data is read, and the result goes through the
normal supervisor.actions.split, so it is undoable like any manual split.

Split out of #1398 at the maintainer's request; #1398 now carries only the
packaging changes.

What it does

• Default algorithm is ISO-SPLIT (the one MountainSort uses) when isosplit
  is installed. It is non-parametric — it decides the number of subclusters by
  testing each candidate split for unimodality — so it does not assume clusters
  are Gaussian. Bursting and drifting units are not, which is where a plain
  Gaussian mixture tends to over-split.
• A Gaussian mixture (BIC-selected) is kept as a fallback, and for the case
  where you want to impose the number of subclusters yourself.

Keyboard shortcuts

Shortcut	Action	Behaviour
alt+k	recluster	ISO-SPLIT, number of subclusters chosen automatically
shift+alt+k	recluster_n	Gaussian mixture into a number of subclusters you enter
ctrl+alt+k	recluster_dip	ISO-SPLIT at a dip threshold you enter

Dependencies

Optional, in a new recluster extra:

recluster = [
    "scikit-learn>=1.9.0",
    "isosplit==0.1.4",
]