Add CTest categories, shader & packaging validation, and GitHub Actions CI foundation

[ercmine]

Motivation

• Establish a professional, CMake-aligned test and CI foundation so every commit/PR runs unit tests, headless smoke validation, shader compilation checks, and packaging checks.
• Provide clear blocking vs informational gates, fast local workflows that mirror CI, and reproducible dependency/bootstrap behavior for CI.
• Make shader validation and package layout checks deterministic and visible in CI logs to speed triage.

Description

• Register unit tests and a new headless smoke executable with CTest (labels: unit, headless) and add helper targets render_test_unit and render_test_headless for convenient local invocation via CMake/CTest.
• Add shader validation targets and policy: render_shaders / render_shader_check, RENDER_REQUIRE_SHADER_COMPILATION toggle, auto-discovery of a built shaderc target, and shader compilation logging to out/build/<preset>/logs/shader_compilation.log.
• Add packaging staging and validation via render_package_stage and render_package_validate, which verify expected binaries, shader outputs, and produce a portable archive (.tar.gz or .zip) while emitting out/build/<preset>/logs/package_validation.log for debugging.
• Add CMake presets (linux-ci) and RENDER_BGFX_BUILD_TOOLS option to allow CI to build bgfx tools when required, and wire the validation targets into the top-level render_validate_all umbrella target.
• Add cross-platform GitHub Actions workflows (.github/workflows/pr-validation.yml and main-validation.yml) with a pragmatic matrix (Linux full lane runs shader+packaging, macOS/Windows run build+unit+headless), dependency caching, and artifact upload for Linux lanes.
• Ship developer tooling and docs: scripts/run_validations.sh / .ps1, docs/testing.md, docs/ci.md, and docs/build.md updates that map local commands to CI behavior.

Testing

• Added automated test registrations (CTest) for existing unit suites and a new tests/headless/headless_validation.cpp smoke executable, but no CTest cases were executed in CI as part of this change yet.
• Performed a local configure attempt with cmake --preset linux-debug -DRENDER_ALLOW_FETCHCONTENT=ON, which failed in this environment because FetchContent attempted to clone https://github.com/libsdl-org/SDL.git and the network fetch was blocked (CONNECT tunnel failed, response 403), so shader/tool bootstrapping and full configure did not complete.
• The repository now includes the CMake/CTest targets and CI workflows required to run the following automated checks (once CI or networked bootstrap runs succeed): unit tests via ctest -L unit, headless smoke via ctest -L headless, shader compilation via cmake --build --target render_shader_check, and packaging validation via cmake --build --target render_package_validate.

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Codex Task