Show calendar date/time in the world HUD; base speed 3 in-game min/sec

Bump the engine submodule to 4b91b60 (calendar time of day) and display
the current date and time in the world HUD (День N, HH:MM) instead of just
the day number. Set the day length to 480 scaled seconds so the base (x1)
speed runs at 3 in-game minutes per real second.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Mods/Core/Languages/ru/ui.json

@@ -1,5 +1,6 @@
 {
-  "hud.world": "LittleSim | сид {0} | день {1} | {2} FPS",
+  "hud.world": "LittleSim | сид {0} | {1} | {2} FPS",
+  "hud.datetime": "день {0}, {1:00}:{2:00}",
   "hud.controls": "WASD — камера, колесо — зум, ` — консоль, F1 — инспектор",
   "hud.paused": "ПАУЗА",