feat(bgm): support role= on code block info strings

Parse role.type#id from the fence info string and merge it into each
parsed object, erroring on conflicts with the content. id on the info
string can't combine with $variants. Auto-named blocks now hash their
content instead of the info string, so identical blocks dedupe.

packages/bgm/src/collect.test.ts

@@ -86,4 +86,18 @@ describe('collectPackages', () => {
     defMap.defs.set(tokensKey.replace('tokens.yaml', 'dup.yaml'), [tokens[0]!]);
     expect(() => collectPackages(defMap, fixtureRoot)).toThrow(/Duplicate part/);
   });
+
+  it('throws when an info-string id combines with $variants', () => {
+    const defMap = loadDefs('', fixtureRoot);
+    // A part whose `id` comes from $variants rows, but with an info-string id.
+    const tokensKey = [...defMap.defs.keys()].find((k) => k.endsWith('parts/tokens.yaml'))!;
+    const tokens = defMap.defs.get(tokensKey)!;
+    const variant = {
+      ...tokens[0]!,
+      value: { ...tokens[0]!.value, id: undefined, $variants: './parts/seats.csv' },
+      role: { role: 'part', type: 'token', id: 'wood' },
+    };
+    defMap.defs.set(tokensKey.replace('tokens.yaml', 'dup.yaml'), [variant]);
+    expect(() => collectPackages(defMap, fixtureRoot)).toThrow(/can't combine with \$variants/);
+  });
 });