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),
     },

engine/unit_manager.py

@@ -51,17 +51,22 @@ class UnitManager:
         return count
 
     def refill_ammo(self):
-        """Randomly refill ammo during gameplay."""
+        """Randomly refill ammo during gameplay.
+
+        Per-frame refill probabilities are scaled by the current difficulty's
+        weapon_refill_multiplier (harder difficulties refill less often).
+        """
         import random
+        multiplier = getattr(self.game, "weapon_refill_multiplier", 1.0)
         for ammo_type, data in self.game.ammo.items():
             if ammo_type == "bomb":
-                if random.random() < 0.02:
+                if random.random() < 0.02 * multiplier:
                     data["count"] = min(data["count"] + 1, data["max"])
             elif ammo_type == "mine":
-                if random.random() < 0.05:
+                if random.random() < 0.05 * multiplier:
                     data["count"] = min(data["count"] + 1, data["max"])
             elif ammo_type == "gas":
-                if random.random() < 0.01:
+                if random.random() < 0.01 * multiplier:
                     data["count"] = min(data["count"] + 1, data["max"])
 
     def spawn_gas(self, parent_id=None):

rats.py

@@ -45,6 +45,12 @@ class MiceMaze:
         self.difficulty = config.DEFAULT_DIFFICULTY
         self.initial_rat_count = config.BASE_INITIAL_RATS
         self.rat_speed_multiplier = 1.0
+        # Difficulty-driven gameplay parameters (defaults; overridden by _apply_difficulty).
+        self.gas_spread_speed = 50
+        self.weapon_refill_multiplier = 1.0
+        self.max_babies = 3
+        self.mate_stop_male = 100
+        self.mate_stop_female = 200
         self._apply_difficulty(self.profile_integration.get_setting('difficulty', config.DEFAULT_DIFFICULTY), persist=False)
         
         self.cell_size = 40
@@ -176,6 +182,12 @@ class MiceMaze:
             int(round(config.BASE_INITIAL_RATS * difficulty_config["starting_rats_multiplier"])),
         )
         self.rat_speed_multiplier = difficulty_config["speed_multiplier"]
+        # Difficulty-driven gameplay parameters.
+        self.gas_spread_speed = difficulty_config.get("gas_spread_speed", 50)
+        self.weapon_refill_multiplier = difficulty_config.get("weapon_refill_multiplier", 1.0)
+        self.max_babies = difficulty_config.get("max_babies", 3)
+        self.mate_stop_male = difficulty_config.get("mate_stop_male", 100)
+        self.mate_stop_female = difficulty_config.get("mate_stop_female", 200)
         if persist:
             self.profile_integration.set_setting("difficulty", normalized_difficulty)
 

units/gas.py

@@ -13,7 +13,7 @@ class Gas(Unit):
         super().__init__(game, position, id, collision_layer=CollisionLayer.GAS, unit_type=UnitType.GAS)
         self.parent_id = parent_id
         # Specific attributes for gas
-        self.speed = 50
+        self.speed = getattr(game, "gas_spread_speed", 50)
         self.fight = False
         self.age = 0
         if parent_id:

units/rat.py

@@ -228,10 +228,10 @@ class Male(Rat):
     def fuck(self, unit):
         if not unit.pregnant:
             self.game.render_engine.play_sound("SEX.WAV")
-            self.stop = 100
+            self.stop = getattr(self.game, "mate_stop_male", 100)
-            unit.stop = 200
+            unit.stop = getattr(self.game, "mate_stop_female", 200)
             unit.pregnant = PREGNANCY_DURATION
-            unit.babies = random.randint(1, 3)
+            unit.babies = random.randint(1, getattr(self.game, "max_babies", 3))
 
 class Female(Rat):
     def __init__(self, game, position=(0,0), id=None):