Multi-block child containers inside proved sibling units still reduce to one wrapper-list fragment

What happened

After proving sibling complex units where each list-item already contained:

• a paragraph
• a nested list
• a quote

the next honest question was whether those child containers could themselves become multi-block:

• nested list with two items
• quote with two paragraphs

What fixed it

It still did not need a new fragment seam.

Selection across sibling units still reduced cleanly to:

• one bulleted-list fragment
• containing already-proved sibling list-item units

That survived:

• slate-v2 extraction
• slate-dom-v2 clipboard transport
• Chromium copy/paste

Why this works

The model still holds because the fragment unit is unchanged.

The nested list and quote each got internally broader, but the outer sibling unit is still one list-item.

So the higher-level fragment story stays:

• proved sibling unit
• lifted into one wrapper list of proved sibling units

Reusable rule

When testing whether the wrapper-list model still holds:

• widen child containers before inventing a new outer fragment abstraction
• if the outer sibling unit is still stable, try to prove the round-trip before designing a new seam

Related issues

• 2026-04-05-multiple-complex-list-units-still-reduce-to-one-wrapper-list-fragment-when-each-unit-stays-proved.md