Phase A: wire climate + day/night into the world; design doc

Bump the engine submodule to a684c23 (Climate + day/night ambient). The
world scene now registers UseClimate and UseDayNight, so it darkens at
night and the HUD shows the season and temperature alongside the date.
Add docs/растения.md capturing the whole plant-ecosystem design (growth as
a suitability product, hybrid Mendelian genome, climate, lighting with
shadows, reproduction, persistence) and the A-D phase plan.

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

Mods/Core/Languages/en/ui.json

@@ -1,6 +1,11 @@
 {
   "hud.world": "LittleSim | seed {0} | {1} | {2} FPS",
   "hud.datetime": "day {0}, {1:00}:{2:00}",
+  "hud.climate": "{0} {1}°C",
+  "season.spring": "spring",
+  "season.summer": "summer",
+  "season.autumn": "autumn",
+  "season.winter": "winter",
   "hud.controls": "WASD — camera, wheel — zoom, ` — console, F1 — inspector",
   "hud.paused": "PAUSED",