feat(rng-seam): SeededRandomSource mirrors the engine's two System.Random streams

SVSim.BattleEngine/Rng/SeededRandomSource.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace SVSim.BattleEngine.Rng
+{
+    // Default source. Faithfully reproduces the engine's own RNG: BattleManagerBase seeds two separate
+    // System.Random(RandomSeed) instances (_stableRandom synced, _stableRandomOnlySelf self-only) at
+    // BattleManagerBase.cs:721-722. The authoritative server uses this; tests use ScriptedRandomSource.
+    public sealed class SeededRandomSource : IRandomSource
+    {
+        private readonly Random _synced;
+        private readonly Random _self;
+
+        public SeededRandomSource(int seed)
+        {
+            _synced = new Random(seed);
+            _self = new Random(seed);
+        }
+
+        public double NextUnit() => _synced.NextDouble();
+        public int NextSelf(int max) => _self.Next(max);
+    }
+}