feat(bgm): resolve relative part assets against source dir

packages/bgm/src/collect.ts

@@ -13,6 +13,7 @@
  *
  * See docs/bgm-format.md for the format's concrete behavior.
  */
+import * as path from 'node:path';
 import picomatch from 'picomatch';
 import { collectVirtualFiles } from './markdown.js';
 import { parseDefText, readDefFiles } from './parse.js';
@@ -218,7 +219,13 @@ function asPackage(def: ParsedDef, source: string): PackageDef {
 
 function asPart(obj: Record<string, unknown>, source: string): Part {
   try {
-    return validatePart(obj) as unknown as Part;
+    const part = validatePart(obj) as unknown as Part;
+    // Resolve relative asset paths against the directory of the source file
+    // (path-style name relative to the games root, e.g. `harbor/parts/`).
+    // Real files may carry a leading slash from an empty root; strip it.
+    const dir = path.posix.dirname(source).replace(/^\/+/, '');
+    part.baseUrl = dir ? `${dir}/` : '';
+    return part;
   } catch (err) {
     throw wrapZod(err, source);
   }