Adds geom::context (Context with marks/locks, Boolean flag, notify
callbacks, own model size / zone tolerance; MarkLock; MassPropertyBehavior;
create/get_or_create_context), Kernel::context, and wires the
UNPORTED(geom.context) SmTol context overloads (Option<&Context>).
Difftest: 2013 cases, all bit-identical.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
bherbruck