[draft] Raise diff boundary for strict schemas via a matchNodes predicate

[YousefED]

Problem

In suggestion mode, a node-type change (e.g. paragraph → heading) is diffed as a leaf-level delete old + insert new. Inside a strict container the tombstoned old child ends up next to the inserted new child — two block-contents in one parent. For schemas like BlockNote's blockContainer (content blockContent blockGroup?) that's schema-invalid and throws Invalid content for node blockContainer.

Solution

This POC lets the diff decide modify-in-place vs. replace via a caller-supplied predicate, so the boundary can be raised to a whole node — producing two sibling blockContainers (allowed by blockContainer+) instead of two block-contents in one.

• lib0/delta.diff gains an optional matchNodes(a, b) argument. The diff already pairs a removed node with an inserted node by name to decide "same node → descend / else → replace"; this just makes that one decision overridable. Defaults to name-equality, so existing behaviour is unchanged.
• syncPlugin gains a matchNodes option, forwarded to the PM→Y write diff. Declaring a node's identity (e.g. "a blockContainer is identified by its first block-content child") makes a child-type change report the two containers as different → whole-node replace → two siblings. A plain text edit keeps the same identity → descends and merges as before.

The binding stays schema-agnostic — what identifies a node is the integrator's policy.

syncPlugin({
  matchNodes: (a, b) =>
    a.name === b.name &&
    (a.name !== 'blockContainer' || firstBlockChildName(a) === firstBlockChildName(b))
})

Caveats

• The BlockNote demo doesn't work yet as it needs some changes to handle conflicting duplicate IDs. We'll create a separate POC there
• This impacts the "mergeability" of certain changes, which needs to be tested. In particular:
• -- the POC is tested for the "change block type" scenario, but not yet created to also handle <blockgroup> (nesting) related cases
• -- changes like "edit text + change from paragraph to heading" won't merge nicely

Tests

tests/block-boundary.test.js — strict BlockNote-like schema; verifies the boundary raise renders two sibling blockContainer

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