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.ts

@@ -21,6 +21,7 @@ import { validatePackage, validatePart, validateSetup, validateSurface } from '.
 import { expandVariants } from './variants.js';
 import {
   BgmError,
+  ROLES,
   type DefFile,
   type ParsedDef,
   type Package,
@@ -31,8 +32,6 @@ import {
   type Surface,
 } from './types.js';
 
-const ROLES = new Set<Role>(['package', 'part', 'surface', 'setup']);
-
 /** Every definition parsed from a def file, keyed by its path-style name. */
 export interface DefMap {
   /** All def files (real + virtual), keyed by name. */
@@ -174,6 +173,11 @@ class PackageAcc {
   }
 
   private add(role: Role, def: ParsedDef, fileName: string) {
+    // `id` on the info string can't combine with `$variants`, since every
+    // row supplies its own `id` and would override it.
+    if (def.role?.id && '$variants' in def.value) {
+      throw new BgmError(`id on the info string can't combine with $variants`, fileName);
+    }
     const expanded = this.expand(def.value, def.file, def.source);
     for (const obj of expanded) {
       switch (role) {