Add lightweight render pass orchestration (render graph) and shell integration

[ercmine]

Motivation

• The renderer had grown many ad-hoc stages (shadows, main lit, post, UI, debug, VFX) and required an explicit, inspectable, engine-owned pass orchestration layer.
• Implement a practical, extensible frame composition model that makes pass order, resource producers/consumers, and dependencies first-class and testable without introducing an enormous AAA scheduler.

Description

• Added a lightweight render-pass registry with explicit resources, pass definitions, execute/enable hooks, execution context, diagnostics, and a textual dump_graph() in engine/render/render_pass_system.hpp and engine/render/render_pass_system.cpp.
• Implemented validation and ordering logic that checks for duplicate names, missing dependencies/resources, write conflicts, read-without-producer (unless imported), and cycle detection and resolves a deterministic topological execution order using declared depends_on plus inferred producer→consumer edges.
• Introduced an explicit resource model (RenderResourceDesc) supporting RenderTarget, DepthTarget, ShadowMap, PostProcessTexture, UiTarget, and imported Backbuffer, and a resource-usage model (RenderPassResourceUsage) with Read/Write/ReadWrite intents.
• Integrated the pass system into the runtime shell (engine/shell/main.cpp) so the frame is now assembled through named passes: shadow-pass, main-lit-pass, bloom-extract-pass, bloom-blur-pass, bloom-composite-pass, outline-pass, ui-pass, debug-pass, and present-pass, including declared resources and explicit dependencies.
• Wire-in per-pass CPU timing into existing debug tooling via RendererPassTiming and track active/inactive passes via RenderPassDiagnostics.
• Added unit coverage in tests/render/render_pass_system_tests.cpp validating pass registration, execution ordering, enable/disable behavior, missing-producer detection, and cycle detection.
• Updated build files and docs: added the new sources to the render_renderer library, added the render_render_pass_system_tests target and CTest entry, and documented the system and current frame layout in docs/rendering.md and engine/render/README.md.

Files created or updated (high level): engine/render/render_pass_system.hpp, engine/render/render_pass_system.cpp, tests/render/render_pass_system_tests.cpp, engine/shell/main.cpp, CMakeLists.txt, docs/rendering.md, engine/render/README.md.

Testing

• Added a unit test target render_render_pass_system_tests which exercises ordering, validation, enable/disable behavior, and cycle detection and is intentionally deterministic and headless; the test source lives at tests/render/render_pass_system_tests.cpp.
• Attempted to configure and build with cmake --preset linux-debug and cmake --build --preset linux-debug --target render_render_pass_system_tests render_shell; configuration/build failed in this environment due to unresolved external dependencies (SDL3 and bgfx/bx/bimg), so the new unit test was not executed here.

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Codex Task