Contract 6.26.0

The game family, after the Elixir Gym's first run on it (feedback #125-#127).

• game_events selects the events reads made inside the window's own instants, not the UTC dates around them (#125). A window holding no read returns nothing, and timezone now matters. game_days_read lists the game days a read covered, a note names the game days in the window nothing read, and running_on_latest_day is described (#126).
• The events read is anchored to the 10:00Z board-day like the daily leaderboards, instead of 24 hours after the last one (#126). It had drifted about 26h50m apart and skipped a game day about every ten days (2026-09-12, 2026-09-22).
• game_clock({at: 'YYYY-MM-DD'}) means that game day: its start, 10:00Z. Before, it meant 00:00Z, which is still the day before on the game's grid, and a note now says which instant was used (#127).

Additive: game_days_read; corrected: game_events' window, game_clock's date-only at.