StatechartGraph: Pick<Graph, "now">

Capability slice handed to statechart.

Closes #364: the statechart capture is a snapshot of the current configuration computed from graph.now alone. The full Graph parameter that #257 left in place handed a getter commit, derived, input, hydrate, and every other mutating handle — a structural lie when the implementation reads exactly one property. Per §12.3 the parameter exposes only now.

Narrowing is type-level. A real Graph is still assignable, so call sites keep working; the discipline is enforced at compile time inside the implementation.