fix(sports): honour every pending kickoff in the idle back-off, not just the first (#772)

_note_scheduled_start_candidate kept one kickoff. While it was inside its
15-minute grace every later kickoff was refused, and by the time the grace
ended the later one had passed and was refused again as already past. So of
two favourites kicking off within 15 minutes of each other, the second lost
its own grace: if the first game was not live by then (a rain delay, a
postponement, ESPN slow to flip it) and ESPN had not flipped the second
either, the back-off went straight back to its ceiling and the second game
was noticed up to that late.

Later kickoffs now wait in a short queue (_later_scheduled_starts, the
earliest 8). When the current kickoff's grace ends, the earliest queued one
still inside its own grace takes over -- including one that has already
passed. A kickoff still holds the live cadence for at most its own grace, so
a postponed game costs the same quarter of an hour as before, and
_next_scheduled_start_ts keeps its meaning for anything that reads or sets
it. The promotion is a module function, so the mixin's method set is
unchanged.

Table tests replay the idle loop on a fake clock over kickoff schedules;
mutation-checked (the old code fails 10 of the new tests).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -194,6 +194,20 @@ Internal; no behaviour change. Stage 3 of `docs/RUN_LOOP_REDESIGN.md`.
   (`refresh_registry_in_background()`, backing off for a minute after an
   offline failure), so a later load has them. The store, install and update
   paths still fetch as before.
+- A sports live manager's idle back-off now honours every pending kickoff,
+  not just the first. `_note_scheduled_start_candidate()` kept one kickoff
+  and, while it was inside its 15-minute grace, refused every later one; by
+  the time the grace ended the later one had passed and was refused again.
+  So of two favourites kicking off within 15 minutes of each other, the
+  second lost its own grace: if the first game was not live by then (a rain
+  delay, a postponement, ESPN slow to flip it) and ESPN had not flipped the
+  second either, the back-off went back to its ceiling and the second game
+  was noticed up to the ceiling (15 minutes by default) late. Later
+  kickoffs now wait in a short queue (`_later_scheduled_starts`, the
+  earliest 8) and each takes over with a grace of its own when the one
+  before it expires. A kickoff still
+  holds the live cadence for at most its own grace, so a postponed game
+  costs the same quarter of an hour as before.
 
 ### ESPN date-range fetches: fewer requests, fewer at once