docs: say that the board has two shapes now

F1 described "one lane per game" as a fact about the timeline, which is
now false half the time, and the prefs key space did not list the pref
that decides it. Both say why the merged board sorts with
endingSoonestFirst rather than a bare end date, since that is the part a
later change is most likely to "simplify".

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

docs/PRD.md

@@ -66,8 +66,8 @@ no stored key. A game may have several sources; see `docs/INGESTION.md` § Three
 ### Features
 
 **F1 — Timeline view.**
-A horizontal timeline, one lane per game, spanning a scrollable date range with "today" pinned as a
-vertical marker. Each event is a bar from `startsAt` to `endsAt`. Bars are colored by game, and
+A horizontal timeline, by default one lane per game, spanning a scrollable date range with "today"
+pinned as a vertical marker. Each event is a bar from `startsAt` to `endsAt`. Bars are colored by game, and
 completed events render at reduced opacity with a check. Clicking a bar opens a detail panel with
 title, type, exact start/end in the user's local timezone, source link, and a completion toggle.
 
@@ -81,6 +81,20 @@ calendar on screen, and nothing left saying which day, whose game, or which even
 six-week bar starts weeks off-screen, so a name that rides off with its own start date leaves a
 coloured rectangle behind.
 
+**The reader chooses how it stacks.** Lanes keep a game's events adjacent and comparable, which is
+what makes the board readable for someone playing four of them — but that reader also has one queue
+of deadlines, and lanes scatter it: what ends tonight sits three lanes below what ends next month,
+and no scroll position puts them side by side. So a pair of pills in the board's header switches
+between **By game** and **Ending soonest**, and the choice is remembered (`prefs.timelineGroup`,
+default `"game"` — the board every existing reader already has). Merged, the order is
+`endingSoonestFirst`, the same comparator behind the list's "Ending soonest" (F2), so the two views
+cannot mean different things by the same words and an unannounced end still sorts behind every dated
+one rather than claiming a place in the queue. Merged there is also no lane heading to say whose
+event a bar is, and hue alone cannot answer that once thirteen games share a stack — so each bar
+carries its game's short name, with the full name in its tooltip. A bar too narrow for a tag and a
+title both keeps the title: a chopped game name reads as a broken word, and the hue and the tooltip
+still answer it.
+
 **The reader sets the scale.** A patch cycle is six weeks and a login campaign can run for months,
 so no single density answers both "what am I in the middle of this week?" and "how do the next three
 months line up?". A pair of controls steps through a ladder of day widths, and the choice is