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

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

SVSim.BattleEngine/Rng/HeadlessBattleMgr.cs

@@ -0,0 +1,42 @@
+using Wizard.BattleMgr;
+
+namespace SVSim.BattleEngine.Rng
+{
+    // The headless authoritative single-battle mgr. Overrides the three BattleManagerBase RNG methods
+    // (now virtual per the Task-4 DP5 patch) to delegate to an injected IRandomSource instead of the
+    // IsForecast-gated System.Random fields. This is the F2 decoupling: VFX stays suppressed
+    // (IsForecast == true) while RNG rolls real. The skill RNG path calls BattleManagerBase.GetIns()
+    // .StableRandom*, and the base ctor registers `this` as the singleton, so constructing the battle as
+    // HeadlessBattleMgr makes every roll dispatch (virtually) to these overrides.
+    //
+    // randomResult is set inside the overrides (it has a protected setter, reachable only from a
+    // subclass — NOT from RandomSourceBridge); it is read by the Phase-2 NetworkSkill_cost_change emit
+    // path, so the overrides keep it faithful. The arithmetic itself lives in RandomSourceBridge so it
+    // stays unit-testable and reusable by a future NetworkBattleManagerBase-derived mgr.
+    public sealed class HeadlessBattleMgr : SingleBattleMgr
+    {
+        private readonly IRandomSource _rng;
+
+        public HeadlessBattleMgr(IBattleMgrContentsCreator contentsCreator, IRandomSource rng = null)
+            : base(contentsCreator)
+        {
+            _rng = rng ?? new SeededRandomSource(contentsCreator.RandomSeed);
+        }
+
+        public override int StableRandom(int val)
+        {
+            double unit = _rng.NextUnit();
+            randomResult = unit;
+            return RandomSourceBridge.Range(val, unit);
+        }
+
+        public override double StableRandomDouble()
+        {
+            double unit = _rng.NextUnit();
+            randomResult = unit;
+            return unit;
+        }
+
+        public override int StableRandomOnlySelf(int val) => _rng.NextSelf(val);
+    }
+}

SVSim.BattleEngine/Rng/RandomSourceBridge.cs

@@ -4,7 +4,7 @@ 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
+    // engine by the parity test (RngSeamTests.Default_source_matches_engine_generator_and_formula). Mirrors
     // BattleManagerBase.StableRandom: `(int)Math.Floor((double)val * unit)`.
     public static class RandomSourceBridge
     {