Slice 1 of dependency-chain display: TaskRowViewModel gains DependsOnTaskId
plus the extrinsic ShowAsChainMember/ChainStep/ChainAfterLabel contract for
Slice 2's rail/badge rendering. Regroup's ClassifyItems walks each row's
DependsOnTaskId chain per section, pulls dependents directly under their
head regardless of SortOrder, and falls back to a flat row + label when the
head isn't in the same section (mirrors the ParentInView precedent).
Planning children never join a chain group - parent indent wins - and only
ever carry the label.
No AXAML changes; that's Slice 2.
