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The Selection holds Entries, not Segments

Reverses 0010. That ADR is superseded, not rewritten — its body stays as it was written.

Context​

0010 ruled the Selection onto SegmentId because a Segment was a drawn piece of one Entry, with no identity of its own before that ADR gave it a stable id. Selecting the whole Entry could not say "this piece, not that one," so the Selection had to hold the finer unit.

#421 removes the premise. What 0010 called a Segment is now an ordinary child Entry, with its own authored EntryId, its own parentId, and no stored classification of its own (Q17, ruled 2026-09-17). A row source rule says which segmented parent draws its children on its own row; the children themselves are Entries the store already indexes, reads, writes and undoes through the one door every other Entry uses. SegmentId named a unit with no EntryId behind it. Once every piece a Gantt draws is an Entry, SegmentId names nothing a consumer authored, and a Selection keyed on it holds an id no dataset.entries door ever hands back.

The spike found the cost of keeping two id spaces before the ADR was written. S4 audited every site that assumes a row's drawn units and the Selection's held units are the same list (SPIKE-FINDINGS.md, Q9). Nine sites read entryIds[0] across seven files. Seven of them ask "the row's subject," and two ask "the first selected Entry" — gantt-shell.ts:1480 and segment-selection.ts:170, SegmentSelection.step(). With several child Entries selected on one segmented row, step() read only the first of the nine and moved it, leaving the rest of the Selection's own members untouched by the same keyboard gesture that had just moved one of them. C3 fixed the nine sites as one audit (999f599), and this ADR is why the fix reads "Entries," not "Segments," on the far side of it: the unit a keyboard command steps and the unit a Selection holds are now the same EntryId, because there is no second id to disagree with it.

Decision​

The Selection is a set of EntryId. gantt.selectedEntryIds replaces gantt.selectedSegmentIds. beforeSelectionChange and selectionChange report { from, to } over Entries. view/entry-selection.ts replaces view/segment-selection.ts, holding the same two jobs 0010 gave the module it replaces: the ids themselves, and the pane rule that decides what a pointer hit adds to them.

The pane rule is unchanged in shape, narrowed in unit. A click in the grid pane still selects every Entry the row owns — the row's subject and, on a segmented row, every child it draws as a bar. A click on the timeline still selects the one Entry under the pointer. Ctrl-click and shift-range still collect Entries the same way they collected Segments. targetUnder still owns the question, and nothing downstream asks it twice.

CommandTarget keeps one set, not two. 0010 gave a command both segmentIds and a derived entryIds, because a Segment and its owning Entry were different things a command might need separately — Delete read the finer unit, Lock read the coarser one. There is no finer unit left. A command reads entryIds alone, and entries.remove(id) is the one removal door: an Entry that was a Segment removes the same way an Entry that was never drawn as one does.

Consequences​

  • SegmentId, Item.segmentId, FrameBar.segmentIds, FrameRow.segmentIds, selectableSegmentsInRowOrder and the data-segment-id stamp retire with the type ADR 0026 retires. This ADR does not retire them on its own; it states the Selection's new unit, and 0026 states the removal.
  • The Picked Item, which 0010 deleted, stays deleted. It existed only because the Selection could not hold a Segment. Nothing in this reversal brings it back: an Entry was always a unit the Selection could hold, so the pick-versus-select split 0010 closed does not reopen.
  • "What paints as selected is what moves" still holds, for the same reason 0010 gave it. The paint and the gesture read one set, and that set is now EntryId end to end — the frame's FrameBar carries the Entry id it draws, the pane rule adds Entry ids, and the diff that decides what repaints runs over Entry ids.
  • The hot path keeps its budget (I5). The selection diff runs over Entries instead of Segments, and it stays O(what changed).
  • A rolled-up Entry is not a special case. 0010 needed one, because an Entry with no Segment of its own could not be selected by clicking its bar — it drew none. A segmented parent draws no bar of its own either (Q26), and it needs none: its row click still selects every child Entry the row shows, through the same pane rule that already reads a row's Entry list.
  • Undo is unaffected. The Selection was never part of a ChangeSet, under either ADR. What changes is which id space a consumer reads back from beforeSelectionChange.

Supersedes 0010 in full. Its premise — that a drawn piece needs an id space of its own because it has no EntryId — no longer holds once every drawn piece is an Entry. Issue #421.