Migrate Build System from Autotools to CMake

[somethingwithproof]

Summary

As part of the '2026 Enterprise Ready' roadmap, Spine should transition its build system from legacy Autotools (configure.ac, Makefile.am) to CMake.

Work

Why Migrate?

1. Modern Tooling: CMake trivially generates compile_commands.json, which is required for modern IDEs (clangd), advanced static analysis (CodeQL, clang-tidy), and secure build tooling.
2. Speed: CMake, especially when paired with the Ninja backend, evaluates the build graph and parallelizes compilation significantly faster than standard make.
3. Developer Experience: Autotools (m4 macros) represents significant technical debt and a high barrier to entry for new contributors. CMake is the undisputed industry standard for C/C++ projects.
4. Testing Integration: CTest provides a structured, modern framework for running tests (including Valgrind) directly from the build system, simplifying our CI pipelines.

Implementation Plan

1. Write a CMakeLists.txt that mirrors the exact feature detection logic currently in configure.ac (finding libmysqlclient, libnetsnmp, libssl, checking for cap_t, etc.).
2. Implement install rules and CTest targets for existing tests.
3. Update GitHub Actions workflows to exclusively use the CMake path (removing the fallback logic).
4. Run CMake side-by-side with Autotools for a deprecation period, then remove bootstrap, configure.ac, and Makefile.am.

Acceptance Criteria

• Work items are implemented and validated.
• Changes preserve behavior unless explicitly intended.