ci: validate every member with mcpp 2026.10.1.1; latest_mcpp -> 2026.10.1.1

[Sunrisepeak]

Moves validate.yml's MCPP_VERSION and index.toml's latest_mcpp to mcpp 2026.10.1.1. That release is published and in the xim index: openxlings/xim-pkgindex#915 is merged, and the published index states latest -> 2026.10.1.1.

The full sweep runs every member on every platform with it.

• Descriptor grammar: the release changes none, so min_mcpp is unchanged.
• Narration on standard error: mcpp's narration now goes to standard error. tests/run_members.sh captures both streams, so its checks are unaffected.
• Host modules: a host module of an index package (the mcpp.plugins rules, for example) is now compiled once per key and kept in the global cache. Every member with a build program exercises this path.