Merge Down from main: bring r26.1.0 up to date with main's accumulated content before the SDP-1399 EBF (SDP-1397 fix, doc-only fixes, deprecated_files.txt additions, new SysConfig.adoc guide, parse_sysctl.pl improvements).
See SessionLog-2026-09-10.md for the full accounting of what this includes and why it's safe to ship.