ADR-008: Stack Dependencies — From-Source Builds for C++ Dependencies

[Ozaq]

Description

Proposes ADR-008: adopting the stack-dependencies repository as the standard mechanism for provisioning approved third-party C++ dependencies from source across all ECMWF build environments (CI VMs, CI on HPC, HPC production).

Key points:

• Dependencies pinned as git submodules at specific version tags
• Transitionary adoption — dependency-by-dependency migration
• Orthogonal to package managers (Spack/Conan/vcpkg) — produces self-contained source bundles
• Governance lifecycle for adding, updating, and retiring dependencies
• Permanent exemptions for system-level dependencies (compilers, MPI, OS libs)

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.