feat(api): add missing block/item placeholder textures

- Add mod_assets for weaveloader.api:missing_block and missing_item
- CopyModAssets target to deploy placeholders to mods/WeaveLoader.API/assets/
- WorldIdRemap: use placeholder icons instead of bedrock/apple

WeaveLoader.API/WeaveLoader.API.csproj

@@ -13,4 +13,12 @@
     <AppendRuntimeIdentifierToOutputPath>false</AppendRuntimeIdentifierToOutputPath>
   </PropertyGroup>
 
+  <!-- Mod textures: use Target so they only copy when API builds to mods/, not when copied as dependency -->
+  <Target Name="CopyModAssets" AfterTargets="Build">
+    <MakeDir Directories="$(OutputPath)assets\blocks" />
+    <MakeDir Directories="$(OutputPath)assets\items" />
+    <Copy SourceFiles="mod_assets\blocks\missing_block.png" DestinationFolder="$(OutputPath)assets\blocks\" SkipUnchangedFiles="true" />
+    <Copy SourceFiles="mod_assets\items\missing_item.png" DestinationFolder="$(OutputPath)assets\items\" SkipUnchangedFiles="true" />
+  </Target>
+
 </Project>

WeaveLoader.API/mod_assets/README.md

@@ -0,0 +1,10 @@
+# WeaveLoader.API Mod Assets
+
+Placeholder textures for the missing block and missing item (unresolved mod content).
+Stored in `mod_assets/` to avoid conflict with the `Assets/` C# source folder (Windows case-insensitivity).
+Copied to `assets/blocks/` and `assets/items/` in the build output.
+
+- **Blocks:** `mod_assets/blocks/missing_block.png` → icon `weaveloader.api:missing_block`
+- **Items:** `mod_assets/items/missing_item.png` → icon `weaveloader.api:missing_item`
+
+These are used when a world contains blocks or items from mods that are no longer installed.