feat(timeline): let a lane be one deadline queue, started or not

`endingSoonestFirst` holds every unstarted event behind every running
one. That is right for a checklist — you cannot do a thing that has not
opened — and it is one answer of two for a board, which is asked what
runs out first. An event opening on Friday and closing on Sunday is a
nearer deadline than one running now until October, and that order can
never show it.

So `byDeadline` is the same comparator with that clause removed, and
`timelineLanes` takes a `split` flag choosing between them. It defaults
to the old behaviour, so nothing moves yet.

The flag is also the one case where lane mode reorders inside a lane,
which the module otherwise refuses to do. Deliberate: the given order is
live-first whatever the reader chose in the list, so honouring it would
draw exactly the block this was asked not to draw.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/shared/time.ts

@@ -344,6 +344,30 @@ export function endingSoonestFirst(
   return a.clock.msRemaining - b.clock.msRemaining;
 }
 
+/**
+ * Sort key: soonest deadline first, and nothing else.
+ *
+ * The other half of the timeline's "put them all together" (PRD F1). Where
+ * `endingSoonestFirst` holds every unstarted event behind every running one —
+ * which is what a *checklist* wants, since you cannot do a thing that has not
+ * opened — this asks the one question a Gantt board is for: what runs out
+ * first. An event opening on Friday and closing on Sunday is a nearer deadline
+ * than one running now until October, and a reader looking at a calendar rather
+ * than a to-do list is entitled to see it in that order.
+ *
+ * The `endsAt: null` rule is the same in both: an unannounced end is real but is
+ * not a deadline, so it sorts behind every dated row rather than claiming a
+ * place in the queue.
+ */
+export function byDeadline(
+  a: { clock: EventClock },
+  b: { clock: EventClock },
+): number {
+  if (a.clock.msRemaining === null) return b.clock.msRemaining === null ? 0 : 1;
+  if (b.clock.msRemaining === null) return -1;
+  return a.clock.msRemaining - b.clock.msRemaining;
+}
+
 /**
  * A plain-language caption for an event's window.
  *