ci: validate every member with mcpp 2026.10.1.2; latest_mcpp -> 2026.10.1.2

[Sunrisepeak]

Moves validate.yml's MCPP_VERSION and index.toml's latest_mcpp to mcpp 2026.10.1.2. That release is published and in the xim index: openxlings/xim-pkgindex#916 is merged, and the published index states latest -> 2026.10.1.2.

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

• Descriptor grammar: the release changes none, so min_mcpp is unchanged.
• BMI placement: a dependency's BMIs now lie below its package's directory in the build directory, and a unit reads one module map of what it reaches (A shared member's compile commands depend on the selected members when another member compiles a module of the same name, so -p and --workspace recompile it in turn mcpp-community/mcpp#751). No check here reads a BMI path.
• Position independence: every unit of an ELF target is compiled with -fPIC, so each member is compiled once more and its global cache entries are refilled.
• Pack and test: mcpp pack states its build, and [build] jobs bounds test and pack (mcpp pack reports its build as planning: the pack's ninja drive has no progress report mcpp-community/mcpp#753).