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/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.