Implement manifest-driven shader pipeline with runtime library, hot-reload, and metadata

[ercmine]

Motivation

• Replace ad-hoc shader handling with a canonical, engine-owned shader pipeline that scales to many programs, stages and variants.
• Provide deterministic include resolution, shared include files, backend-aware compilation, and engine-facing reflection metadata for future material/binding validation.
• Support a safe development workflow with hot-reload plus CI-friendly build graph integration so runtime and packaging use the same generated outputs.

Description

• Added a manifest-driven compilation flow and CMake integration that compiles per backend/category/program/variant/stage, emits stage JSON and program.json metadata, and stages outputs under <build>/bin/shaders/bin and <build>/bin/shaders/metadata; see cmake/ShaderCompilation.cmake and cmake/compile_shaders.cmake and shaders/shaders.cmake.
• Introduced a clean source layout and include rules under shaders/ (shared shaders/includes/, shaders/materials/, shaders/post/, shaders/debug/) and small example shaders and include files demonstrating conventions and predictable include resolution (*.sh, *.def.sc).
• Implemented an engine-owned runtime API ShaderProgramLibrary (engine/render/shader_library.{hpp,cpp}) with ShaderProgramId, backend-aware path resolution, safe load_program, and timestamp-based reload_if_stale hot-reload that preserves the prior program on failure.
• Emitted deterministic reflection metadata (stage JSON with SHA256, program.json with stage list/refs), wired packaging/validation to require metadata, updated render_shell to use ShaderProgramLibrary, and added a focused unit test tests/render/shader_pipeline_tests.cpp validating path resolution and backend mapping.

Testing

• Executed the pipeline script in a shaderc-unavailable mode: cmake -DRENDER_SOURCE_DIR=/workspace/render -DRENDER_SHADER_OUTPUT_ROOT=/tmp/render_shader_out -DRENDER_SHADER_MANIFEST=/workspace/render/shaders/shaders.cmake -DRENDER_BGFX_SHADERC= -DRENDER_REQUIRE_SHADER_COMPILATION=OFF -DRENDER_SHADER_BACKENDS=spirv -P cmake/compile_shaders.cmake, which ran and returned the expected warning path (skipped compilation) and successfully generated no-op logs.
• Attempted a full CMake configure (cmake --preset linux-debug -DRENDER_ALLOW_FETCHCONTENT=ON) to validate build/test integration, but it failed in this environment due to external network/FetchContent (SDL3) being blocked; this prevented running the full build and CTest in this sandbox.
• Added render_shader_pipeline_tests and wired it into the unit test suite (CTest) so CI can run the new path-resolution tests when dependencies and network/fetch are available; test compilation/run was not completed here because the full build could not finish in this environment.

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Codex Task