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.Simulation/SimulationOptions.cs

@@ -31,6 +31,12 @@ public sealed class SimulationOptions
     /// </summary>
     public string SavesDirectory { get; set; } = "saves";
 
+    /// <summary>
+    /// Directory of mod packs (<c>core</c> and optional add-ons). Relative paths are resolved
+    /// against the content root.
+    /// </summary>
+    public string ModsDirectory { get; set; } = "mods";
+
     /// <summary>
     /// How often a running school writes its clock to disk. Create, pause, speed and shutdown
     /// write immediately; the tick itself never does.