NEO-48: fix in-memory TrySetSkillXpTotal zero on absent skill

Setting XP to 0 is a no-op success when no row exists, matching Postgres DELETE
semantics and TrySetSkillXpTotals behavior.

server/NeonSprawl.Server.Tests/Game/Skills/InMemoryPlayerSkillProgressionStoreTests.cs

@@ -0,0 +1,23 @@
+using Microsoft.Extensions.DependencyInjection;
+using NeonSprawl.Server.Game.Skills;
+using Xunit;
+
+namespace NeonSprawl.Server.Tests.Game.Skills;
+
+public sealed class InMemoryPlayerSkillProgressionStoreTests
+{
+    [Fact]
+    public void TrySetSkillXpTotal_WhenXpZeroAndSkillAbsent_ShouldReturnTrue()
+    {
+        // Arrange
+        using var factory = new InMemoryWebApplicationFactory();
+        var store = factory.Services.GetRequiredService<IPlayerSkillProgressionStore>();
+
+        // Act
+        var applied = store.TrySetSkillXpTotal("dev-local-1", "intrusion", xp: 0);
+
+        // Assert
+        Assert.True(applied);
+        Assert.False(store.GetXpTotals("dev-local-1").ContainsKey("intrusion"));
+    }
+}

server/NeonSprawl.Server/Game/Skills/InMemoryPlayerSkillProgressionStore.cs

@@ -82,7 +82,8 @@ public sealed class InMemoryPlayerSkillProgressionStore(IOptions<GamePositionOpt
         {
             if (xp == 0)
             {
-                return inner.TryRemove(sid, out _);
+                inner.TryRemove(sid, out _);
+                return true;
             }
 
             inner[sid] = xp;