Initial commit: LegacyForge mod loader for Minecraft Legacy Edition

SKSE-style external mod loader with zero game source modifications.
- LegacyForge.Launcher: C# console app that injects runtime DLL into game process
- LegacyForgeRuntime: C++ DLL with PDB symbol resolution, MinHook function hooking, and .NET CoreCLR hosting
- LegacyForge.Core: C# mod discovery and lifecycle management
- LegacyForge.API: Fabric-style mod API with namespaced string IDs, fluent property builders, and event system
- ExampleMod: Sample mod demonstrating block/item registration

LegacyForge.API/Entity/EntityDefinition.cs

@@ -0,0 +1,16 @@
+namespace LegacyForge.API.Entity;
+
+/// <summary>
+/// Fluent builder for defining entity properties.
+/// </summary>
+public class EntityDefinition
+{
+    internal float WidthValue = 0.6f;
+    internal float HeightValue = 1.8f;
+    internal int TrackingRangeValue = 80;
+
+    public EntityDefinition Width(float width) { WidthValue = width; return this; }
+    public EntityDefinition Height(float height) { HeightValue = height; return this; }
+    public EntityDefinition TrackingRange(int range) { TrackingRangeValue = range; return this; }
+    public EntityDefinition Size(float width, float height) { WidthValue = width; HeightValue = height; return this; }
+}

LegacyForge.API/Entity/EntityRegistry.cs

@@ -0,0 +1,38 @@
+namespace LegacyForge.API.Entity;
+
+/// <summary>
+/// Represents an entity type that has been registered with the game engine.
+/// </summary>
+public class RegisteredEntity
+{
+    public Identifier StringId { get; }
+    public int NumericId { get; }
+
+    internal RegisteredEntity(Identifier id, int numericId)
+    {
+        StringId = id;
+        NumericId = numericId;
+    }
+}
+
+/// <summary>
+/// Entity registration via the LegacyForge registry.
+/// Accessed through <see cref="Registry.Entity"/>.
+/// </summary>
+public static class EntityRegistry
+{
+    public static RegisteredEntity Register(Identifier id, EntityDefinition definition)
+    {
+        int numericId = NativeInterop.native_register_entity(
+            id.ToString(),
+            definition.WidthValue,
+            definition.HeightValue,
+            definition.TrackingRangeValue);
+
+        if (numericId < 0)
+            throw new InvalidOperationException($"Failed to register entity '{id}'.");
+
+        Logger.Debug($"Registered entity '{id}' -> numeric ID {numericId}");
+        return new RegisteredEntity(id, numericId);
+    }
+}