feat(rng-seam): HeadlessBattleMgr override + decoupling/parity tests (F2 resolved)

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

SVSim.BattleEngine.Tests/RngSeamTests.cs

@@ -1,6 +1,8 @@
 using System;
 using NUnit.Framework;
 using SVSim.BattleEngine.Rng;
+using Wizard;
+using Wizard.Battle;
 
 namespace SVSim.BattleEngine.Tests
 {
@@ -51,5 +53,48 @@ namespace SVSim.BattleEngine.Tests
             Assert.That(src.NextSelf(99), Is.EqualTo(3));
             Assert.That(() => src.NextSelf(99), Throws.InvalidOperationException, "should throw on self overrun");
         }
+
+        // The decoupling (F2): the override must roll REAL values even though IsForecast == true (which
+        // forces the un-overridden engine methods to return 0). A ScriptedRandomSource proves the value
+        // came from the injected source, not the engine's zeroing.
+        [Test]
+        public void Override_rolls_real_values_under_IsForecast()
+        {
+            HeadlessEngineEnv.EnsureInitialized();
+            BattleManagerBase.IsForecast = true; // would zero the un-overridden engine RNG
+
+            // 3 units; with RandomSourceBridge.Range(val, unit) = floor(val*unit):
+            //   StableRandom(7) with 0.5  -> floor(3.5) = 3
+            //   StableRandomDouble()      -> 0.25
+            //   StableRandomOnlySelf(10)  -> scripted self pick 4
+            var src = new ScriptedRandomSource(new[] { 0.5, 0.25 }, new[] { 4 });
+            var mgr = new HeadlessBattleMgr(new HeadlessContentsCreator(), src);
+
+            Assert.That(mgr.StableRandom(7), Is.EqualTo(3), "StableRandom did not use the injected source");
+            Assert.That(mgr.randomResult, Is.EqualTo(0.5), "StableRandom must set randomResult to the rolled unit");
+            Assert.That(mgr.StableRandomDouble(), Is.EqualTo(0.25), "StableRandomDouble did not use the injected source");
+            Assert.That(mgr.randomResult, Is.EqualTo(0.25), "StableRandomDouble must set randomResult");
+            Assert.That(mgr.StableRandomOnlySelf(10), Is.EqualTo(4), "StableRandomOnlySelf did not use the injected source");
+        }
+
+        // Parity: with the DEFAULT (seeded) source, HeadlessBattleMgr.StableRandom must equal what the
+        // verbatim engine would compute — floor(val * new System.Random(seed).NextDouble()) — pinning the
+        // re-authored RandomSourceBridge arithmetic to the engine's own formula+generator. (The default
+        // source seeds from HeadlessContentsCreator.RandomSeed == 12345.)
+        [Test]
+        public void Default_source_matches_engine_generator_and_formula()
+        {
+            HeadlessEngineEnv.EnsureInitialized();
+            BattleManagerBase.IsForecast = true;
+
+            var mgr = new HeadlessBattleMgr(new HeadlessContentsCreator()); // default SeededRandomSource(12345)
+            var reference = new System.Random(12345);
+
+            for (int i = 0; i < 10; i++)
+            {
+                int expected = (int)System.Math.Floor(7 * reference.NextDouble());
+                Assert.That(mgr.StableRandom(7), Is.EqualTo(expected), $"parity drift at roll {i}");
+            }
+        }
     }
 }