ENH: Add block argument to stc.plot() (gh-14105)

[CedricConday]

Closes #14105.

What

Adds block=False to mne.viz.plot_source_estimates, threaded through
SourceEstimate.plot and VolSourceEstimate.plot_3d.

stc.plot() returns immediately, so in a script there is no opportunity to
interact with the figure before the interpreter exits. raw.plot() has block;
this gives stc.plot() the same option.

The default is False, preserving current behaviour — generating figures in a
script or a report is the common case and must not start blocking.

Scope

Deliberately not added to VectorSourceEstimate.plot / .plot_3d, which
document from plot_vector_source_estimates. Only the two methods that share
the plot_source_estimates docstring accept it, per @cbrnr's "YAGNI and let's
just fix this one" and @larsoner's preference not to add block everywhere.

VolSourceEstimate.plot_3d had to be included because it shares that docstring
via copy_function_doc_to_method_doc — documenting the parameter without
accepting it fails test_docstring_parameters (PR02: Unknown parameters).

Implementation

The coregistration GUI already had this logic inline:

if block and self._renderer._kind != "notebook":
    _qt_app_exec(self._renderer.figure.store["app"])

Rather than copy it, it is now _qt_block in mne/viz/backends/_utils.py, and
_coreg.py calls that. So this removes a duplicate rather than adding one. The
matplotlib backend blocks through the existing plt_show(True, block=block).

Tests

• test_qt_block_runs_event_loop — arms a QTimer that records and quits, then
  asserts the recorder ran, so an early return is caught. State-based rather
  than a wall-clock threshold, to avoid CI flakiness.
• test_qt_block_without_qt_app — notebook backend, and a renderer whose
  plotter came from the caller, both return without attempting to block.
• test_process_clim_plot gains a block="yes" → TypeError check.

Verified locally, including end-to-end under Xvfb on the testing dataset:
stc.plot(block=True) returned after 2.55 s against a 2.5 s quit timer, with the
timer confirmed fired, and still returned a Brain. Removing the bool
validation makes stc.plot(block="yes") block forever, which is what that check
prevents. mne/viz/backends/tests/ and mne/tests/test_docstring_parameters.py
pass (29 passed, 6 skipped, 1 xfailed); ruff format and ruff check are clean.

Blocking on the notebook backend is not tested — no notebook backend available
in the environment I verified in.

AI assistance disclosure

Per CONTRIBUTING's policy on AI assistance: this contribution is AI-assisted. The
code, tests and this description were written by Claude (Anthropic), directed by
me. I set the scope, made the design calls above, reviewed the diff, and ran the
verification described. It is not the output of a fully automated tool — every
step was directed and reviewed by me before submission.