docs: the board's boundary heading, and the two readings of it

F1 described bars that carry their own not-started marking and stopped
there. It now has the label naming where the running ones end, and the
choice between keeping that block and dropping it for one deadline order
— including why the second is a re-sort rather than a hidden heading,
which is the part a future change would otherwise get wrong.

The heading itself shipped a commit earlier without this; same feature,
so it is written up once here rather than split across two entries.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/DATA-MODEL.md

@@ -219,7 +219,7 @@ Namespaced, versioned, and small. Nothing here ever goes to the server.
 "gacha-tracker:v1:ignored"      // { [eventId]: { at } }  — "stop showing me this"
 "gacha-tracker:v1:prefs"        // { region, hiddenGames[], knownGames[]?, focusGame, sort, view,
                                 //   timelineDayWidth, timelineGroup, timelineUpcoming,
-                                //   detectDaily, showCompleted,
+                                //   timelineSplitUpcoming, detectDaily, showCompleted,
                                 //   showIgnored, theme, regionConfirmed, onboarded }
                                 // timelineDayWidth is px per day on the board, stored as the
                                 // measurement rather than a step number and read through
@@ -234,6 +234,11 @@ Namespaced, versioned, and small. Nothing here ever goes to the server.
                                 // make room for things nobody can do yet. Set from settings
                                 // alongside showCompleted / showIgnored, and the only one of
                                 // the three scoped to one view. See PRD F1.
+                                // timelineSplitUpcoming keeps those events in their own block
+                                // under a "Not started yet" heading (true, the default) or puts
+                                // them in one deadline order with the running ones (false).
+                                // Only read when timelineUpcoming is on, but stored either way
+                                // so switching that back on restores the answer given. PRD F1.
                                 // knownGames is every lane the reader has been offered. Absent
                                 // means unrecorded, not "offered nothing" — see PRD F8; a lane
                                 // missing from it is new to them and arrives switched off.