Answer the stability review: guard the argument lookup, and correct the claims the split made false

src/markdown/block-directive-arguments.ts

@@ -1,11 +1,13 @@
 import type { BlockType } from '../adf/block-directives.ts'
 
-const blockArguments: Readonly<Record<string, string>> = {
-  blockTaskItem: 'state',
-  panel: 'panelType',
-  taskItem: 'state',
-} satisfies Partial<Record<BlockType, string>>
+const argumentByType = new Map(
+  Object.entries({
+    blockTaskItem: 'state',
+    panel: 'panelType',
+    taskItem: 'state',
+  } satisfies Partial<Record<BlockType, string>>),
+)
 
 export function blockArgument(type: string): string | undefined {
-  return blockArguments[type]
+  return argumentByType.get(type)
 }