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Outnumbered.Tests/ProgressionModelTests.cs

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+using Outnumbered.Domain;
+using Xunit;
+
+namespace Outnumbered.Tests;
+
+// ProgressionModel: XP curve + prestige boost. Expected XP values are INDEPENDENT literals (computed offline from the
+// documented formula), so these catch an implementation typo, not just lock current output.
+public class ProgressionModelTests
+{
+    // XpToNext = LevelXpBase + round(LevelXpStep * (max(0,L-1))^LevelXpExponent); defaults Base=100, Step=8, Exp=1.7.
+    [Theory]
+    [InlineData(1, 100)]   // (L-1)=0 -> 0 -> just the base
+    [InlineData(2, 108)]
+    [InlineData(3, 126)]
+    [InlineData(5, 184)]
+    [InlineData(10, 435)]
+    [InlineData(25, 1876)]
+    [InlineData(50, 6076)]
+    [InlineData(100, 19855)]
+    public void XpToNext_matches_curve(int level, long expected) =>
+        Assert.Equal(expected, ProgressionModel.XpToNext(level, T.Prog()));
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-5)]
+    public void XpToNext_floors_negative_level_at_base(int level) =>
+        Assert.Equal(100L, ProgressionModel.XpToNext(level, T.Prog())); // Math.Max(0, L-1) guards the Pow base
+
+    [Fact]
+    public void XpToNext_is_strictly_increasing_above_level_one()
+    {
+        var c = T.Prog();
+        long prev = ProgressionModel.XpToNext(1, c);
+        for (int l = 2; l <= c.LevelCap; l++)
+        {
+            long cur = ProgressionModel.XpToNext(l, c);
+            Assert.True(cur > prev, $"XpToNext({l})={cur} should exceed XpToNext({l - 1})={prev}");
+            prev = cur;
+        }
+    }
+
+    // PrestigeXpMultiplier = 1 + prestige * (PrestigeXpBoostPercent/100); default 10% per prestige.
+    [Theory]
+    [InlineData(0, 1.00)]
+    [InlineData(1, 1.10)]
+    [InlineData(5, 1.50)]
+    [InlineData(10, 2.00)]
+    public void PrestigeXpMultiplier_matches(int prestige, double expected) =>
+        T.Close(expected, ProgressionModel.PrestigeXpMultiplier(prestige, T.Prog()));
+
+    [Fact]
+    public void Retune_pinned_prestige_boost_is_ten_percent() =>
+        Assert.Equal(10, T.Prog().PrestigeXpBoostPercent); // pins the balance-retune knob
+}