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
+        }
+    }
+}