Scale gas spread, weapon refill, litter size, and post-mating stop with difficulty

Four gameplay parameters are now driven by the selected difficulty,
following the existing 'hard = harder for the player' philosophy:

  param                       easy   normal   hard
  gas_spread_speed            40     50       90     (higher = slower gas)
  weapon_refill_multiplier    1.3    1.0      0.6    (less ammo on hard)
  max_babies                  2      3        5      (more pups on hard)
  mate_stop_male / female     120/240 100/200 60/120 (shorter stop on hard)

Implementation:
- engine/config.py: new keys on each DIFFICULTY_OPTIONS entry.
- rats.py: _apply_difficulty() loads them onto the game (with __init__
  defaults for safety); normal keeps the previous hardcoded values.
- units/gas.py: Gas.speed reads game.gas_spread_speed (fallback 50).
- engine/unit_manager.py: refill_ammo() scales per-frame refill chances
  by game.weapon_refill_multiplier (fallback 1.0).
- units/rat.py: Male.fuck() uses game.mate_stop_male/mate_stop_female
  and random.randint(1, game.max_babies) (fallbacks to old values).

All reads use getattr with the previous constant as fallback, so older
configs / saved state keep working.

engine/config.py

@@ -27,6 +27,15 @@ DIFFICULTY_OPTIONS = (
         "label": "Easy",
         "starting_rats_multiplier": 1,
         "speed_multiplier": 1.0,
+        # Gas spread tick interval (higher = slower spread = less effective weapon).
+        "gas_spread_speed": 40,
+        # Multiplier applied to per-frame weapon refill probabilities.
+        "weapon_refill_multiplier": 1.3,
+        # Upper bound for babies per litter (random.randint(1, max_babies)).
+        "max_babies": 2,
+        # Stop ticks after mating for the male / the pregnant female.
+        "mate_stop_male": 120,
+        "mate_stop_female": 240,
         "fill": (235, 246, 234),
         "accent": (88, 148, 82),
     },
@@ -35,6 +44,11 @@ DIFFICULTY_OPTIONS = (
         "label": "Normal",
         "starting_rats_multiplier": 2,
         "speed_multiplier": 1.5,
+        "gas_spread_speed": 50,
+        "weapon_refill_multiplier": 1.0,
+        "max_babies": 3,
+        "mate_stop_male": 100,
+        "mate_stop_female": 200,
         "fill": (252, 242, 223),
         "accent": (204, 146, 44),
     },
@@ -43,6 +57,11 @@ DIFFICULTY_OPTIONS = (
         "label": "Hard",
         "starting_rats_multiplier": 3,
         "speed_multiplier": 2.0,
+        "gas_spread_speed": 90,
+        "weapon_refill_multiplier": 0.6,
+        "max_babies": 5,
+        "mate_stop_male": 60,
+        "mate_stop_female": 120,
         "fill": (251, 229, 229),
         "accent": (188, 68, 68),
     },