V2 mixed-inline clipboard proofs should span top-level blocks before broader tree shapes

What happened

After the one-block mixed-inline proof went green, there was an obvious trap:

• it would be easy to declare mixed-node clipboard “done”
• while the first real document-level case was still unproved

That missing case was:

• partial first block
• partial last block
• real block break in the fragment
• inline children preserved on both sides

What fixed it

The proof expanded one step, not ten:

• slate-v2 mixed-inline fragment extraction now spans multiple top-level blocks when each touched block still matches the current proved mixed-inline shape
• mixed-inline fragment insertion now accepts multi-block fragments into the current proved mixed-inline target shape
• slate-dom-v2 clipboard boundary proof now round-trips that fragment
• the browser suite now proves the same case in Chromium with a dedicated two-block route

That keeps the scope honest:

• broader than one block
• still far narrower than arbitrary nested block trees

Why this works

The first useful generalization is the smallest one that adds real document pressure.

For clipboard semantics, that means crossing a block boundary while preserving the inline structure we already proved inside each block.

Jumping straight from one-block mixed-inline to arbitrary nested trees would blur two different questions:

1. does the fragment model survive a real multi-block document cut?
2. does the model survive arbitrary tree shapes?

Only the first one needed to be solved now.

Reusable rule

When expanding slate-v2 proof scope:

• move from one proved shape to the next real document pressure case
• do not skip directly to arbitrary tree support unless the narrower document seam is already green

If the next proof can be stated as “same shape, but across a real document boundary,” do that before inventing a bigger abstraction story.

Related issues

• 2026-04-03-slate-v2-clipboard-boundary-proof-must-split-fragment-semantics-and-dom-transport.md
• 2026-04-04-v2-editable-blocks-need-structure-preserving-dom-reconciliation-for-mixed-inline-editing.md