Add HSchool.Content project for JSONC definitions, catalog, and map validation. Update solution structure to include new content and tests projects. Enhance school management to support mod packs and map instances, ensuring proper loading and validation. Revise documentation to reflect these changes and update tests for new functionality.

src/HSchool.Content/IContentLog.cs

@@ -0,0 +1,17 @@
+namespace HSchool.Content;
+
+/// <summary>Warnings from the loader — last-wins collisions, not fatal errors.</summary>
+public interface IContentLog
+{
+    void Warning(string message);
+}
+
+/// <summary>Drops warnings. Tests that do not care about collisions use this.</summary>
+public sealed class NullContentLog : IContentLog
+{
+    public static NullContentLog Instance { get; } = new();
+
+    public void Warning(string message)
+    {
+    }
+}