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

SVSim.BattleEngine.Tests/RngSeamTests.cs

@@ -17,5 +17,23 @@ namespace SVSim.BattleEngine.Tests
             Assert.That(RandomSourceBridge.Range(3, 0.5), Is.EqualTo(1));   // floor(1.5)   = 1 (middle of 3)
             Assert.That(RandomSourceBridge.Range(1, 0.5), Is.EqualTo(0));   // floor(0.5)   = 0
         }
+
+        // SeededRandomSource(seed) must reproduce the engine's own generators EXACTLY: BattleManagerBase
+        // seeds both _stableRandom and _stableRandomOnlySelf as `new System.Random(RandomSeed)`
+        // (BattleManagerBase.cs:721-722). NextUnit() == synced.NextDouble(); NextSelf(max) == self.Next(max).
+        [Test]
+        public void SeededSource_reproduces_two_System_Random_streams()
+        {
+            const int seed = 12345;
+            var src = new SeededRandomSource(seed);
+
+            var refSynced = new System.Random(seed); // mirrors _stableRandom
+            var refSelf   = new System.Random(seed); // mirrors _stableRandomOnlySelf (separate stream)
+
+            for (int i = 0; i < 8; i++)
+                Assert.That(src.NextUnit(), Is.EqualTo(refSynced.NextDouble()), $"NextUnit drift at {i}");
+            for (int i = 0; i < 8; i++)
+                Assert.That(src.NextSelf(100), Is.EqualTo(refSelf.Next(100)), $"NextSelf drift at {i}");
+        }
     }
 }

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);
+    }
+}