Refactor: Relocate computeSSAO to SSAOSystem

[MichaelFisher1997]

The computeSSAO method in IRenderContext (src/engine/graphics/rhi.zig) violates interface segregation. It should be moved to a dedicated SSAOSystem.

This is part of the ongoing effort to clean up the RHI interface (continuing from #189).

[opencode-agent]

This is a valid refactoring request that aligns with the existing TODO in the codebase. Here's my analysis:

Classification: Refactoring / Feature Request (continuing from #189)

Current State:

• computeSSAO is defined in IRenderContext.VTable at src/engine/graphics/rhi.zig:288
• Implemented in Vulkan backend at src/engine/graphics/rhi_vulkan.zig:3243 (computeSSAOInternal at line 3165)
• Called from SSAOPass in src/engine/graphics/render_graph.zig:186
• There's already a TODO at rhi.zig:287 referencing issue [Medium] Segregate IRenderContext Interface #189

Pattern to Follow:
The codebase already has ShadowSystem (src/engine/graphics/shadow_system.zig) as a dedicated system pattern that encapsulates:

• Resources (pipelines, render passes, framebuffers, descriptors)
• State management
• Public methods (beginPass, endPass)

Implementation Approach:

1. Create src/engine/graphics/ssao_system.zig following ShadowSystem pattern
2. Move SSAO resources from VulkanContext to SSAOSystem
3. Move computeSSAOInternal logic to SSAOSystem.compute()
4. Update render_graph.zig:186 to call ssaosystem.compute() instead of rhi.context().computeSSAO()
5. Remove computeSSAO from IRenderContext.VTable and related test mocks

Impact: Minimal - this is a pure refactor that improves interface segregation. The functionality remains the same, just reorganized.

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