ci: validate against mcpp 2026.9.25.1; latest_mcpp -> 2026.9.25.1

[Sunrisepeak]

Summary

This moves validate.yml to mcpp 2026.9.25.1 (mcpp-community/mcpp#690, #691) and records it as latest_mcpp in index.toml. min_mcpp does not move, because no descriptor grammar changed.

What 2026.9.25.1 changes for this index:

• A consumer's include_dirs no longer reach the compile lines of its dependencies. No member here declares include_dirs, so no member relied on the old behaviour.
• [build] defines is a set keyed by macro name.
• A member inside an index archive (mcpp = "*/<dir>/mcpp.toml") inherits its archive's workspace. None of the archives here declares an inheritable workspace table.
• The build cache epoch moves from 3 to 4, so the first run is cold.

No descriptor needs a change. This run measures that statement on every member and platform.

Test plan

• Before the release, 16 members passed mcpp test -p with the release candidate on Linux: cjson, zlib, brotli, c-ares, expat, libpng, sqlite3, pcre2, spdlog-compiled, fmtlib.fmt, yaml-cpp, xxhash, md4c, libffi, mimalloc and yyjson.
• Full workspace matrix on this pull request.