Fix scene-object registration interfering with other COMPAS backends

[arpastrana]

Summary

compas_plotter registered its scene objects eagerly at import time. That side effect broke scene-object resolution for every other COMPAS visualization backend (the Viewer and the Notebook) whenever compas_plotter happened to be imported first.

This PR removes the eager registration and instead exposes the plugin the way compas_viewer does, via __all_plugins__, so COMPAS discovers it through the standard lazy collect_all path.

The bug

Note the script never imports compas_plotter.

from compas.datastructures import Mesh
from compas.geometry import NurbsCurve
from compas_viewer import Viewer

mesh = Mesh.from_meshgrid(dx=10, nx=10)
arch = NurbsCurve.from_points([[5, 0, 0], [5, 5, 5], [5, 10, 0]])  # the trigger

viewer = Viewer()
viewer.scene.add(mesh)   # -> SceneObjectNotRegisteredError: ... context: Viewer
viewer.scene.add(arch)
viewer.show()

Root cause

COMPAS lazily initialises its scene-object registry with a one-shot guard in
compas/scene/context.py:

def get_sceneobject_cls(item, **kwargs):
    if not ITEM_SCENEOBJECT:          # only fires while the registry is empty
        register_scene_objects()      # the collect_all pluggable — ALL backends
    ...

The register_scene_objects pluggable uses the collect_all selector, so this single call is what registers every backend (base COMPAS, Viewer, Notebook, Plotter, …). It only runs while ITEM_SCENEOBJECT is still empty.

The failure chain:

1. NurbsCurve.from_points(...) needs a geometry backend, so COMPAS runs plugin discovery, which imports every installed compas_* package — including compas_plotter.
2. Importing compas_plotter ran register_scene_objects() eagerly (via from . import scene and the eager call at the end of scene/__init__.py), writing ITEM_SCENEOBJECT["Plotter"] = {...}.
3. ITEM_SCENEOBJECT is now non-empty, so COMPAS's if not ITEM_SCENEOBJECT guard never fires. The collect_all discovery that would have registered the Viewer and base contexts is skipped.
4. viewer.scene.add(mesh) can't resolve Mesh in the "Viewer" context and raises SceneObjectNotRegisteredError.

compas_plotter was also missing __all_plugins__, so the PluginManager never imported compas_plotter.scene to find the @plugin-decorated factory the proper way — the eager import was compensating for that, which is exactly what messed up with the guard.

The fix

• Add __all_plugins__ = ["compas_plotter.scene"] in compas_plotter/__init__.py (mirrors compas_viewer).
• Remove the from . import scene side-effect import.
• Remove the eager register_scene_objects() call at the end of compas_plotter/scene/__init__.py.

Registration now happens only through COMPAS's plugin discovery + lazy collect_all, so backends no longer clobber each other's registration.

Verification

Reproduced and fixed against compas 2.15.1, compas_viewer 2.0.2, compas_occ.

• Before: the MRE raises SceneObjectNotRegisteredError on viewer.scene.add(mesh).
• After: the MRE adds both objects with no exception; mesh resolves to MeshObject and arch to NurbsCurveObject.
• compas_plotter standalone still registers and draws (Plotter().add(mesh)
• New regression test test_import_does_not_populate_registry_eagerly asserts,
  in a subprocess, that importing compas_plotter leaves ITEM_SCENEOBJECT empty and that collect_all then registers both the Plotter and base contexts.
• Full suite: 16 passed. ruff check / ruff format --check clean.

[arpastrana]

I updated the changelog, so not sure why the PR checklist does not pass in CI...