Add engine-owned scene system with hierarchy, cameras, lights and renderables (Statement 11)

[ercmine]

Motivation

• Provide the engine-owned spatial foundation (scene graph) that supports stable identity, parent-child hierarchy, local/world transforms, cameras, lights, renderables, visibility flags, and debug names.
• Make the scene the canonical extraction layer for the renderer so future gameplay/tools can build on a stable runtime abstraction.

Description

• Added a new scene module under engine/scene/ exposing Scene, SceneNodeId (index+generation), CameraComponent, LightComponent, RenderableComponent, visibility/layer masks, reparent policies, and debug name APIs (engine/scene/scene.hpp and engine/scene/scene.cpp).
• Implemented stable handle validation and reuse-safety via index+generation, defensive lookup, and generation bump/free-list reuse on destroy.
• Implemented parent/child operations with cycle prevention, explicit ReparentPolicy (KeepLocalTransform / KeepWorldTransform), subtree dirty propagation, and update_world_transforms() for top-down world transform computation.
• Added renderer extraction helpers: build_camera_view(...), collect_visible_renderables(...), and collect_visible_lights(...), and integrated the scene into the shell so render_shell builds a small demo scene and submits visible renderables to the renderer without leaking backend types.
• Added module docs (engine/scene/README.md, docs/scene.md) and updated docs/architecture.md and docs/rendering.md to mention the scene integration, and wired the scene into CMake with a render_scene library and render_scene_tests target.

Testing

• Ran a local compile+unit run for the new scene unit test by compiling tests/scene/scene_tests.cpp and supporting core files with g++ and executing the binary, which succeeded (test assertions passed).
• Built and ran core runtime unit test (tests/core/core_runtime_tests.cpp) via direct g++ invocation and the test succeeded.
• Attempted full CMake configure/build with cmake --preset linux-debug, which failed in this environment due to missing external dependencies (SDL3/bgfx); this is an environment issue and does not affect the scene module unit tests executed above.

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Codex Task