feat(battle-node): BattleSeeds — stable per-battle seed derivation

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.UnitTests/BattleNode/Lifecycle/BattleSeedsTests.cs

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+using NUnit.Framework;
+using SVSim.BattleNode.Lifecycle;
+
+namespace SVSim.UnitTests.BattleNode.Lifecycle;
+
+[TestFixture]
+public class BattleSeedsTests
+{
+    // Golden values pin cross-run/cross-platform stability. They were computed from the exact
+    // splitmix64 mix specified in BattleSeeds. If these ever change, replay reproducibility broke —
+    // do NOT "update them to match"; find what changed the algorithm (e.g. someone slipped in
+    // GetHashCode, which is per-process randomized).
+    [Test]
+    public void Derive_golden_values_are_stable()
+    {
+        Assert.That(BattleSeeds.Stable(12345), Is.EqualTo(1577307848));
+        Assert.That(BattleSeeds.IdxChange(12345, 906243102), Is.EqualTo(1638231407));
+        Assert.That(BattleSeeds.DeckShuffle(12345, 906243102), Is.EqualTo(355953180));
+        Assert.That(BattleSeeds.IdxChange(12345, 847666884), Is.EqualTo(518125159));
+        Assert.That(BattleSeeds.Stable(99999), Is.EqualTo(323349150));
+    }
+
+    [Test]
+    public void Derive_is_deterministic_for_same_inputs()
+    {
+        Assert.That(BattleSeeds.Derive(7, "x", 42), Is.EqualTo(BattleSeeds.Derive(7, "x", 42)));
+    }
+
+    [Test]
+    public void Derive_differs_across_tag_master_and_discriminator()
+    {
+        var baseline = BattleSeeds.Derive(7, "x", 42);
+        Assert.That(BattleSeeds.Derive(8, "x", 42), Is.Not.EqualTo(baseline), "different master");
+        Assert.That(BattleSeeds.Derive(7, "y", 42), Is.Not.EqualTo(baseline), "different tag");
+        Assert.That(BattleSeeds.Derive(7, "x", 43), Is.Not.EqualTo(baseline), "different disc");
+    }
+
+    [Test]
+    public void Derive_is_always_non_negative()
+    {
+        // System.Random tolerates any int, but a non-negative seed keeps parity with prod's
+        // positive seed values and avoids surprises.
+        Assert.That(BattleSeeds.Stable(int.MinValue), Is.GreaterThanOrEqualTo(0));
+        Assert.That(BattleSeeds.Stable(-1), Is.GreaterThanOrEqualTo(0));
+    }
+}