Add forward-plus lighting planner and scene integration (Statement 13) - #13

Motivation

• Provide an engine-owned, forward-plus style lighting planning layer to move beyond naive per-object/per-pixel loops and enable many-local-lights workflows for the stylized Perbug look.
• Surface first-class controls for fog, bloom, directional shadow hints, emissive materials, and outline/highlight hooks in a coherent renderer-facing API.
• Integrate lighting planning with the existing scene, submission, and shell flows so a validation scene can exercise the new data path without yet depending on full GPU pass implementations.

Description

• Added a new lighting module engine/render/lighting.hpp + engine/render/lighting.cpp that defines input structs, GPU-packed output structs, validation helpers, LightingSelectionConfig, LightingFrameData, diagnostics, and build_forward_plus_frame_data which implements a practical forward-plus strategy (camera-relative ranking of point lights, per-view caps, per-object selection).
• Extended the scene model in engine/scene/scene.hpp/scene.cpp with LightComponent::casts_shadows, per-renderable lighting metadata (lit, albedo_tint, emissive_color, emissive_intensity, highlighted), scene-level lighting settings (SceneLightingSettings), and collect_highlighted_renderables + lighting settings accessors.
• Wired extraction and planning in the runtime shell engine/shell/main.cpp to build a stylized validation setup (directional + many point lights, emissive/highlighted renderables, fog/bloom settings) and call build_forward_plus_frame_data each frame to produce deterministic frame data (ready for GPU upload in follow-ups).
• Added deterministic unit tests tests/render/lighting_pipeline_tests.cpp that validate light ranking/capping, per-object light-list limits, diagnostic counts, and settings validation; updated tests/scene/scene_tests.cpp to cover highlight/emissive plumbing; updated CMakeLists.txt and docs (docs/rendering.md, engine/render/README.md) to register tests and describe architecture and deferred work.

Testing

• Ran repository hygiene check: git diff --check passed.
• Added unit test target render_lighting_pipeline_tests and CTest registration for unit.renderer.lighting_pipeline; the test source tests/render/lighting_pipeline_tests.cpp performs deterministic assertions on planner behavior.
• Attempted to configure/build with cmake --preset linux-debug, but full configure/build/test execution is blocked in this environment due to missing third-party SDK roots (SDL3/bgfx), so the new unit test has not been executed here; it is expected to succeed when the project dependencies are available.
• Summary of validation performed: added tests compile-time integration in CMake and local git commit; functional execution of unit tests requires resolved third-party dependencies.

Codex Task