Mod textures, display names, and atlas injection

Mod Atlas (ModAtlas.cpp/h):
- Build merged terrain.png and items.png from mod assets (blocks/*.png, items/*.png)
- Scan vanilla atlas for empty (fully transparent) cells; place mod textures only there
- Install merged atlases over game files before Minecraft::init; restore originals after
- Hook loadUVs to create SimpleIcon objects for mod textures
- Hook registerIcon to return mod icons when requested by name
- FixupModIcons: copy field_0x48 (source-image ptr) from vanilla icons after init

Mod Strings (ModStrings.cpp/h):
- Store mod display names by description ID
- Hook GetString to serve mod names for blocks/items

API changes:
- BlockProperties/ItemProperties: .Name(displayName), namespaced .Icon()
- NativeInterop: displayName params, native_allocate_description_id, native_register_string
- Registry.Assets for string registration
- Output: mods/LegacyForge.API/, mods/ExampleMod/ (per-mod folders)

Mod discovery:
- Scan mods/*/ for mod folders; load DLLs from each
- LegacyForge.API as mod in mods/LegacyForge.API/

ExampleMod:
- Ruby ore block and ruby item with custom textures and names
- Assets: blocks/ruby_ore.png, items/ruby.png, lang files
- Furnace recipe: ruby_ore -> ruby

Runtime: loadUVs, registerIcon, getResourceAsStream, GetString hooks; stb_image for PNG

ExampleMod/ExampleMod.cs

@@ -20,13 +20,15 @@ public class ExampleMod : IMod
                 .Hardness(3.0f)
                 .Resistance(15f)
                 .Sound(SoundType.Stone)
-                .Icon("ruby_ore")
+                .Icon("examplemod:ruby_ore")  // From assets/blocks/ruby_ore.png
+                .Name("Ruby Ore")
                 .InCreativeTab(CreativeTab.BuildingBlocks));
 
         Ruby = Registry.Item.Register("examplemod:ruby",
             new ItemProperties()
                 .MaxStackSize(64)
-                .Icon("ruby")
+                .Icon("examplemod:ruby")  // From assets/items/ruby.png
+                .Name("Ruby")
                 .InCreativeTab(CreativeTab.Materials));
 
         Registry.Recipe.AddFurnace("examplemod:ruby_ore", "examplemod:ruby", 1.0f);