Phantom carrier for the event map a spec contributes.
EMAdd cannot be inferred from a spec's when: { keys: [["chan", "evt"]] } carries
channel and type strings and no payload types, so there is nothing to infer a map from. And
TypeScript has no partial type-argument inference, so a registerSlice<N, St, EMAdd> would
force a caller who names EMAdd to hand-write N and St too.
The way out is to put EMAdd in a **value** position, where inference works. The builders
(defineSlice, defineMiddleware, defineEffect) brand a spec with this
interface, and the register methods read it back with EMAddOf. Nothing exists at
runtime; the property is never assigned.
The property is **required, not optional**: an optional one makes
X extends EventMapCarrier<infer E> match every object and infer unknown. And it is a
*function* type so EMAdd sits in both co- and contravariant position, which keeps the
inference exact rather than widening to a supertype.
Type parameters
