[Medium] Decouple RHI (God Object) into Subsystems

[MichaelFisher1997]

Problem

The RHI struct in src/engine/graphics/rhi.zig violates the Single Responsibility Principle (SRP). It acts as a monolithic facade ("God Object") that handles widely divergent responsibilities:

• Resource Management (Buffers, Textures, Shaders)
• Command Recording & Submission
• UI Rendering
• Device Queries & Synchronization
• Presentation & VSync

This centralization makes the graphics subsystem rigid and difficult to unit test or refactor.

Impacted Files

• src/engine/graphics/rhi.zig
• src/engine/graphics/rhi_vulkan.zig

Action Plan

1. Define Subsystems:
   • ResourceManager: Dedicated to createBuffer, createTexture, destroy*.
   • RenderCommandQueue: Handles beginFrame, endFrame, and submission logic.
   • SwapchainPresenter: Handles surface, vsync, and presentation.
2. Refactor RHI: Change RHI to be a container that holds references to these subsystems, rather than implementing the methods directly via a giant VTable.
3. Update Implementation: Split VulkanContext to align with these new boundaries.

Acceptance Criteria

• RHI struct is significantly smaller or acts only as a composition root.
• Resource creation and command submission are distinct logical paths.

[MichaelFisher1997]

/oc Refactor the monolithic RHI struct in src/engine/graphics/rhi.zig into distinct subsystems: ResourceManager (for buffers/textures), RenderCommandQueue (for frame management), and SwapchainPresenter. Update the Vulkan implementation in rhi_vulkan.zig to reflect this decoupled architecture.

[github-actions]

Created PR #195

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