Problem

We need mixed grouping at the root level.

Solution

After researching the codebase, I believe only 2 options are worth looking at.

Empty block type

PM: empty block is registered as a new block type, in the existing block group, same as paragraph/heading. Its containing blockNode uses its optional blockChildren to hold the grouped children with the desired childrenType.

doc
└── blockChildren (Group)
    ├── blockNode → paragraph
    ├── blockNode → empty // renders nothing
    │              blockChildren (Unordered)
    │                ├── blockNode → paragraph
    │                └── blockNode → paragraph
    └── blockNode → paragraph

Backend: add HMBlockEmpty (or any name) to HMBlockKnownSchema

{
  type: 'Empty',
  id: string,
  attributes: {
    childrenType: HMBlockChildrenType,
    columnCount?: number,
  }
}

Every existing block type already carries childrenType/columnCount for its children. Empty is just a new block type with no rendering of its own.

Pros:

Backend to editor mapping is one to one.
Extra blocks only when needed. Simple docs have zero extra structure.
PM root shape is unchanged.

Cons:

Introduces an invisible block type. Code that iterates or filters by block type needs to remember to exclude Empty.

New root node type

PM: replace root blockChildren with a new node that contains blockChildren+. Each blockChildren has its own listType/columnCount.

doc
└── docRoot
    ├── blockChildren (Group)
    │   └── blockNode → paragraph
    ├── blockChildren (Unordered)
    │   ├── blockNode → paragraph
    │   └── blockNode → paragraph
    └── blockChildren (Group)
        └── blockNode → paragraph

Backend: same HMBlockEmpty block type as Option A. The editor's shape doesn't map 1:1 to the backend tree, needs custom conversion.

Editor to HM block: each blockChildren under docRoot becomes either:
- an array of top level HMBlockNodes (when listType === 'Group')
- one HMBlockEmpty HMBlockNode with attributes.childrenType set, its blockNodes as .children.
HM to editor block: iterate the root HMBlockNode[]. Non Empty blocks become blockChildren with listType="Group". Each Empty block becomes blockChildren with the corresponding listType.

Pros:

Editor blockChildren gains a unified meaning: "always a grouping container." Root and nested behave the same. No special case for root listType.
No invisible block type. Menus, type-based filters, and serializers don't need to know about a hidden wrapper.

Cons:

Conversion layer needs new handling. Non-trivial and needs round-trip tests.

Conclusion

Both are roughly the same amount of code and produce the same backend structure. I don't believe 2nd is much easier to maintain in the editor, so I'd lean towards an empty block type.