Tool contribution model
=======================

A tool session owns runtime state and bindings. ``WorkbenchToolContribution`` exposes
primary views, local actions, and status. ``WorkbenchToolFactory`` composes shipped
contributions. ``ToolViewSelector`` changes the visible layout only.

This keeps Render Toy and ShaderToy alive simultaneously and leaves the workspace as
the document/focus/time owner.

Implementation
--------------

Relevant source: ``ui/WorkbenchToolFactory.*``, ``ui/ToolViewSelector.*``, and
``modes/*/*Session.*``.