ci: validate every member with mcpp 2026.9.26.2

[speak-agent]

Summary

Validate every member with mcpp 2026.9.26.2, released today from mcpp-community/mcpp#702 (issues #699, #701, #703).

What the release changes that a member's build can see:

• the compile database is kept per configuration, and directory names the directory the compiler runs in;
• link flags (ldflags, mcpp::link_flag) are read as words, so a $ORIGIN reaches the linker as written;
• build programs gain mcpp::runtime_search_dir and the prepare action role, and an action role outside source, check, object, artifact and prepare is refused;
• the build-program cache epoch moves from 2 to 3, so each build program runs once more.

Searched before the release: no descriptor here escapes a link flag for the shell or ninja by hand, and the build programs under tests/examples use only the source role. This raise measures that on every member and platform.

index.toml min_mcpp and the openkal pins do not move: no descriptor grammar changed.