Support LLVM 23.1.3 as the native Linux aarch64 default with a managed glibc ecosystem #784

Linux aarch64 needs a native LLVM 23.1.3 default backed by a complete managed glibc runtime and target sysroot.

The implementation covers architecture-specific resources and dependencies, first-run defaults, native GNU targets, module builds, cold installation, packaging, CN mirror validation, and openkal consumers. It also closes the verification gaps found in PR #781. Linux x86_64 retains GCC and existing explicit defaults remain valid.

Design: .agents/docs/2026-10-08-llvm-2313-linux-aarch64-ecosystem-part2-design.md.

Completion requires coordinated index and engine PRs, all relevant CI passing, ecosystem review, release resources on both mirrors, and real SubOS consumer verification.