Interface FieldChangedPayload

FieldChangedEvent — fact-mixin field assignment.

Fired whenever a mixin setter mutates a persistent field that other subsystems may want to observe (Named.setName, Visible.set*Description, Detailed.setDetail / removeDetail, Tangible.setMaterial / setMass, Globbable.setQuantity, etc.).

The payload's field discriminator names the mixin-declared field that changed. PropertyChangedEvent uses a separate KIND with a property discriminator so the property-bag namespace and the fact-mixin field namespace stay distinct on the meta-bus dependency index.

Concrete event classes don't extend a common base — they satisfy the BusEvent<P> structural contract (in api/event.ts) by declaring kind: string + payload: P directly.