Infinite restart loop after Windows 11 Insider Update installation / can't get into safe mode.

* Per subject I installed Windows 11 insider update, accepted the reboot.

* On reboot there was some message about reverting to previous version

* Now it BSODs and reboots in an infinite loop, reporting a problem with FLTMGR.SYS

* If I wait for the window's circle thingy to start before hitting the power button, it reboots before powering down.

* If I power off as soon as the BIOS POST message goes away nothing changes.

Any suggestions as I type this from my Linux partition?