Add engine validation script with attack_coin_status effect handler

- Add attack_coin_status effect handler for coin-flip status conditions
  (e.g., Thunder Shock paralysis on heads)
- Create comprehensive engine_validation.py script (~1250 lines) that
  validates game engine behavior with 29 test cases:
  - Illegal moves (attack without energy, wrong turn, evolution rules)
  - Energy mechanics (attachment limits, cost validation)
  - Weakness calculation (+20 additive mode)
  - Status conditions (paralysis blocks actions, poison damage)
  - Knockout flow (points, forced actions, state cleanup)
  - Win conditions (4 points triggers game over)
- Update game_walkthrough.py Thunder Shock to use new effect handler
- Interactive prompts between sections (Enter to continue, q to quit)
- Uses seed=42 for deterministic, reproducible coin flips

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/core/effects/handlers.py

@@ -445,6 +445,53 @@ def handle_remove_status(ctx: EffectContext) -> EffectResult:
 # See: engine.py _execute_attack() for the TODO on integrating this
 
 
+@effect_handler("attack_coin_status")
+def handle_attack_coin_status(ctx: EffectContext) -> EffectResult:
+    """Flip a coin; on heads, apply a status condition.
+
+    This is the common "flip coin, apply status on heads" pattern used by many
+    attacks like Thunder Shock (paralysis on heads).
+
+    Params:
+        status (str): StatusCondition to apply on heads. Required.
+            One of: poisoned, burned, asleep, paralyzed, confused
+
+    Target:
+        Opponent's active Pokemon.
+
+    Returns:
+        Success with flip result and status application details.
+    """
+    status_str = ctx.get_str_param("status")
+    if not status_str:
+        return EffectResult.failure("No status specified")
+
+    try:
+        status = StatusCondition(status_str)
+    except ValueError:
+        return EffectResult.failure(f"Invalid status: {status_str}")
+
+    flip = ctx.flip_coin()
+    target = ctx.get_target_pokemon()
+
+    if target is None:
+        return EffectResult.failure("No valid target")
+
+    if flip:
+        target.add_status(status)
+        return EffectResult.success_result(
+            f"Flipped heads - applied {status.value}!",
+            effect_type=EffectType.STATUS,
+            details={"flip": "heads", "status": status.value, "target_id": target.instance_id},
+        )
+    else:
+        return EffectResult.success_result(
+            "Flipped tails - no effect",
+            effect_type=EffectType.STATUS,
+            details={"flip": "tails", "status": None},
+        )
+
+
 @effect_handler("coin_flip_damage")
 def handle_coin_flip_damage(ctx: EffectContext) -> EffectResult:
     """Deal damage based on coin flip results.

backend/references/game_walkthrough.py

@@ -255,8 +255,8 @@ def create_card_definitions() -> dict[str, CardDefinition]:
                 name="Thunder Shock",
                 cost=[EnergyType.LIGHTNING],
                 damage=30,
-                effect_id="coin_flip_damage",
-                effect_params={"base_damage": 30, "bonus_on_heads": 0, "apply_paralysis": True},
+                effect_id="attack_coin_status",
+                effect_params={"status": "paralyzed"},
                 effect_description="Flip a coin. If heads, the Defending Pokemon is now Paralyzed.",
             ),
         ],