docs/kernel-contract.md §5 described a design sketch — a fictional
SystemGroup.Play/Edit split, a "field-by-field clone" — written before
Play mode existed. Replaced it with what engine.editor actually does:
IWorld.Snapshot()/Restore() wrapped by IPlayModeController, and
Engine.Host gating Stage.Update on IsPlaying rather than the scheduler
knowing anything about system groups at all.
Also updates the "Frame stages" resolution to record Stage.Present (added
this milestone, not speculative) and the M3 row in the build-order table
to reflect everything actually built: the editor shell, reflection-based
Hierarchy/Inspector, Play/Stop, and the real camera-driven translate
gizmo. README.md's Status section gets the same M3 writeup M0-M2 already
had, in the same voice.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N
