slow area persists as an area rather than instantaneously applying once to enemies in the area

Assets/_Project/Scripts/Gameplay/BuilderSpells/SlowZoneRuntime.cs

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+// Assets/_Project/Scripts/Gameplay/BuilderSpells/SlowZoneRuntime.cs
+using UnityEngine;
+using TD.Core;
+
+namespace TD.Gameplay.BuilderSpells
+{
+    /// <summary>
+    /// Server-only runtime object for a Slow Area cast. Re-scans its radius every frame for the
+    /// rest of the zone's lifetime, continuously refreshing the Cold status on whatever enemies
+    /// currently overlap it.
+    /// </summary>
+    /// <remarks>
+    /// <para><b>Zone, not a snapshot.</b> Replaces <see cref="SlowAreaSpellDefinition"/>'s old
+    /// single <c>OverlapSphereNonAlloc</c> call at cast time, which slowed whoever was standing
+    /// there for a fixed duration regardless of whether they walked out. This ticks continuously
+    /// instead, so only enemies currently inside the radius are ever affected.</para>
+    ///
+    /// <para><b>Refresh, not enter/exit events.</b> Each tick re-applies
+    /// <see cref="EnemyStatus.ApplyEffect"/> with a short fixed window (<see cref="RefreshWindow"/>),
+    /// not the zone's remaining lifetime. An enemy that walks out of the zone simply stops being
+    /// refreshed and the slow expires on its own <see cref="RefreshWindow"/> seconds later —
+    /// no explicit exit detection needed.</para>
+    ///
+    /// <para><b>Not a NetworkObject.</b> It only ever runs where <see
+    /// cref="SlowAreaSpellDefinition.ServerCast"/> runs (the server), and it never needs to exist
+    /// on clients — the zone's visual is spawned independently on every peer via
+    /// <see cref="SlowAreaSpellDefinition.ClientSpawnVfx"/>.</para>
+    /// </remarks>
+    public class SlowZoneRuntime : MonoBehaviour
+    {
+        // Comfortably longer than one frame so a single missed tick doesn't let the slow
+        // visibly drop while an enemy is still inside, but short enough that leaving the
+        // zone reads as an almost-immediate loss of the slow.
+        private const float RefreshWindow = 0.35f;
+
+        private static readonly Collider[] s_overlapBuffer = new Collider[64];
+
+        private float radius;
+        private float slowFactor;
+        private float remainingLifetime;
+        private LayerMask enemyLayerMask;
+        private PlayerSlot owner;
+
+        /// <summary>Positions and configures the zone. Call immediately after AddComponent.</summary>
+        public void Initialize(Vector3 position, float radius, float slowFactor, float lifetime,
+            LayerMask enemyLayerMask, PlayerSlot owner)
+        {
+            transform.position = position;
+            this.radius = radius;
+            this.slowFactor = slowFactor;
+            this.remainingLifetime = lifetime;
+            this.enemyLayerMask = enemyLayerMask;
+            this.owner = owner;
+        }
+
+        private void Update()
+        {
+            remainingLifetime -= Time.deltaTime;
+            if (remainingLifetime <= 0f)
+            {
+                Destroy(gameObject);
+                return;
+            }
+
+            int count = Physics.OverlapSphereNonAlloc(
+                transform.position, radius, s_overlapBuffer, enemyLayerMask);
+
+            for (int i = 0; i < count; i++)
+            {
+                var enemyHealth = s_overlapBuffer[i].GetComponent<EnemyHealth>();
+                if (enemyHealth == null || enemyHealth.IsDead) continue;
+
+                var enemyStatus = s_overlapBuffer[i].GetComponent<EnemyStatus>();
+                if (enemyStatus == null) continue;
+
+                enemyStatus.ApplyEffect(DamageType.Cold, slowFactor, RefreshWindow, owner);
+            }
+        }
+    }
+}