Book and quill cannot be written in: the extra open-book packet sends the client to the read-only view

What happens

A book and quill cannot be written in. The client shows the edit screen for a moment (the Sign button is visible) and then switches to the read-only view with only Done.

The same client on a vanilla 26.3 server keeps the editor open, so this is not the client.

Why

When the player uses a book and quill, the client opens the editor by itself. Pumpkin also sends the open-book packet, so the client opens the book a second time and lands in the read-only view.

Proof: on an instrumented build, disabling only the open-book packet makes writing work. Removing the packet entirely then breaks signed books, which the client does not open on its own. So the packet belongs to the signed book case only.

Steps

1. /give @s writable_book and open it on Pumpkin. The editor flashes and becomes the read-only view.
2. The same on a vanilla 26.3 server keeps the editor open.
3. With the open-book packet disabled, writing works and signed books can no longer be opened.

Notes

Everything else was verified correct while hunting this: the item on the wire is writable_book with id 1371, matching the official 26.3 registry, with no component patch; exactly one packet is sent on use; no inventory, slot or equipment update touches the item; no edit-book packet is lost. A PR with the one-condition gate follows.