initial commit

Outnumbered/Domain/ProgressionModel.cs

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+using Outnumbered.Config;
+
+namespace Outnumbered.Domain;
+
+// XP/level/prestige curves (pure). The stateful level-up loop + its side effects (chat/sound/clan) stay engine-side;
+// this owns only the formulas it walks.
+public static class ProgressionModel
+{
+    // XP required to go from `level` to `level+1`. LevelXpExponent shapes the curve (1 = linear, >1 = accelerating).
+    public static long XpToNext(int level, ProgressionConfig c) =>
+        c.LevelXpBase + (long)Math.Round(c.LevelXpStep * Math.Pow(Math.Max(0, level - 1), c.LevelXpExponent));
+
+    // Cumulative prestige XP boost (prestige never lowers difficulty; it only speeds the climb).
+    public static double PrestigeXpMultiplier(int prestige, ProgressionConfig c) =>
+        1.0 + prestige * (c.PrestigeXpBoostPercent / 100.0);
+}