feat(spec)!: object-gantt markers, object-timeline mapping and both forms' fields take the shape each block reads; five objectui-held contracts reported as forks (#21464, S-objectui-held)

feat(spec)!: object-gantt markers, object-timeline mapping and both forms' fields take the shape each block reads; five objectui-held contracts reported as forks (#21464, S-objectui-held) #6573