Implements IHasHandshakePhase and emits client-shaped InitNetwork/InitBattle/
Loaded/Swap (reacting to the session's pushes) instead of being a passive
TurnEnd-only fixture the session narrates around. This is what lets the
type-agnostic mulligan barrier (next task) work in Scripted mode.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>