ci: validate every member with mcpp 2026.9.30.1; latest_mcpp -> 2026.9.30.1

[Sunrisepeak]

mcpp 2026.9.30.1 (mcpp-community/mcpp#743) is released; its index entry is openxlings/xim-pkgindex#912.

• validate.yml runs every member on every platform with it.
• latest_mcpp moves to it; min_mcpp is unchanged, since the release changes no descriptor grammar.
• The release changes the build's human output again (a package is named when it does work, one status row, Finished from one minute) and restores one mcpp.lock entry per identity. No check in this repository reads that output: tests/openkal/compat.py reads Running bin/, and site-check.yml reads warning:, both unchanged in form.