jsdom contenteditable composition is not a trustworthy IME proof

What happened

The next exact seam after the zero-width renderer policy split was supposed to be simple:

• remove FEFF from the empty-block line-break placeholder path
• prove IME composition still works

We tried to prove it with a jsdom test:

• render an empty editor
• strip the FEFF text node from the line-break placeholder
• dispatch native CompositionEvent start/update/end on the editable root

Why this was not good enough

The event never reached the real Editable composition path in a trustworthy way.

That means a failing jsdom composition test here cannot tell you whether:

• the no-FEFF path is actually broken
• or jsdom/react contenteditable composition is just not simulating the browser path honestly

That is a fake proof seam.

Reusable rule

For IME work in Slate:

• do not treat jsdom contenteditable composition as authoritative unless you have already proved the event path is really reaching the same runtime seam the browser uses

If the harness cannot clear that bar, keep policy conservative and move to a real-browser IME proof instead of pretending a unit test settled it.