Towards the Isabelle2026 (October 2026) release

[Makarius]

Reminder: the release plan is here 
https://sketis.net/2025/plan-for-isabelle2026-october-2026

I have shifted the start with RC1 5 days further ahead: 19-Sep-2026, and the 
critical fork-point 2 weeks later as before, 03-Oct-2026. Thus we will be 
mostly in sync with the AFP fork-point.

So there are now < 2 weeks left to get everything ready for RC1. Then there 
will be precisely 2 weeks to react on feedback by users, on the regular 
isabelle-dev repository.

After 03-Oct-2026, any further changes that are really relevant for the 
release need to be sent to me via e-mail (from "hg export" or "hg bundle").


	Makarius