feat(rng-seam): IRandomSource interface + RandomSourceBridge arithmetic

Adds the RNG seam skeleton (Task 1 of M12): IRandomSource (NextUnit/NextSelf)
and RandomSourceBridge.Range mirroring BattleManagerBase.StableRandom exactly
(`(int)Math.Floor(val * unit)`). RngSeamTests pins the floor arithmetic (1 test, passing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.BattleEngine.Tests/RngSeamTests.cs

@@ -0,0 +1,21 @@
+using System;
+using NUnit.Framework;
+using SVSim.BattleEngine.Rng;
+
+namespace SVSim.BattleEngine.Tests
+{
+    [TestFixture]
+    public class RngSeamTests
+    {
+        // RandomSourceBridge.Range must mirror the engine's exact roll arithmetic:
+        // BattleManagerBase.StableRandom does `(int)Math.Floor((double)val * unit)`.
+        [Test]
+        public void Bridge_Range_mirrors_engine_floor_arithmetic()
+        {
+            Assert.That(RandomSourceBridge.Range(7, 0.0), Is.EqualTo(0));   // floor(7*0)   = 0
+            Assert.That(RandomSourceBridge.Range(7, 0.999), Is.EqualTo(6)); // floor(6.993) = 6 (never == val)
+            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
+        }
+    }
+}

SVSim.BattleEngine/Rng/IRandomSource.cs

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+namespace SVSim.BattleEngine.Rng
+{
+    // The battle RNG seam. The headless authoritative mgr (HeadlessBattleMgr) routes the engine's
+    // StableRandom* calls through this instead of the IsForecast-gated System.Random fields, so the
+    // server rolls real outcomes (decoupling F2) and tests can replay a known sequence.
+    public interface IRandomSource
+    {
+        // Synced stream, [0,1). Drives StableRandom (via RandomSourceBridge.Range) and StableRandomDouble.
+        double NextUnit();
+
+        // Self-only stream, [0,max). Mirrors StableRandomOnlySelf (engine: _stableRandomOnlySelf.Next(val)).
+        int NextSelf(int max);
+    }
+}

SVSim.BattleEngine/Rng/RandomSourceBridge.cs

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+using System;
+
+namespace SVSim.BattleEngine.Rng
+{
+    // The ONE place engine roll-logic is re-authored (the virtual-override seam restates it rather than
+    // body-patching the Engine file). Isolated here so it is unit-testable and pinned to the verbatim
+    // engine by the parity test (RngSeamTests.SeededSource_matches_engine_generator / Task 5). Mirrors
+    // BattleManagerBase.StableRandom: `(int)Math.Floor((double)val * unit)`.
+    public static class RandomSourceBridge
+    {
+        public static int Range(int val, double unit) => (int)Math.Floor((double)val * unit);
+    }
+}