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

packages/bgm/src/collect.test.ts

@@ -25,6 +25,9 @@ describe('collectPackages', () => {
     });
     expect(wood.face).toBe('./assets/tokens.png');
     expect(wood.faceCrop).toEqual([1, 0, 5, 2]);
+    // Relative assets resolve against the source file's directory. The fixture
+    // markdown sits at the games root, so the virtual file is `parts/tokens.yaml`.
+    expect(wood.baseUrl).toBe('parts/');
 
     // Two surfaces: the table board and its child player board.
     expect([...harbor.surfaces.keys()].sort()).toEqual(['board#harbor', 'board#player']);

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);
   }

packages/bgm/src/types.ts

@@ -69,6 +69,11 @@ export interface Part {
   size?: Size;
   /** Fillet radius in mm; defaults to `0`. */
   fillet?: number;
+  /**
+   * Directory of the part's source file (json/yaml/markdown), for resolving
+   * relative asset paths. Set by the loader; not authored.
+   */
+  baseUrl?: string;
   /** Extra fields from the source definition, kept for forwards compatibility. */
   [key: string]: unknown;
 }