Phase 1 / Task 1: Remove computeSSAO from IRenderContext

[MichaelFisher1997]

Goal

Remove the SSAO method from the broad render context and keep SSAO behind its dedicated interface.

Scope

• src/engine/graphics/rhi.zig
• any direct SSAO call sites

Work

• Remove computeSSAO from IRenderContext.VTable
• Remove the IRenderContext.computeSSAO() forwarding method
• Update callers to use rhi.ssao()

Acceptance criteria

• IRenderContext no longer exposes SSAO behavior
• Build passes
• No remaining call sites use the removed API

Parallel-safe

Yes

[github-actions]

This is a Bug/Refactoring task with clear acceptance criteria. The analysis confirms the scope is well-defined.

Findings:

1. IRenderContext.VTable (rhi.zig:687) - has computeSSAO entry that needs removal
2. IRenderContext.computeSSAO() (rhi.zig - no forwarding method exists; the VTable method is called directly)
3. RenderContext.computeSSAO() (rhi.zig:281-283) - forwarding wrapper that needs removal
4. VULKAN_RHI_VTABLE.render.computeSSAO (rhi_vulkan.zig:943) - backend entry that needs removal
5. Test mock (rhi_tests.zig:128,243) - mock computeSSAO that needs removal

No external call sites use IRenderContext.computeSSAO - the SSAO pass in render_graph.zig:285 correctly uses ctx.ssao_ctx.compute() via ISSAOContext, which is the intended design.

Already-correct architecture:

• ISSAOContext (rhi.zig:627-639) with compute() method exists as the dedicated interface
• rhi.ssao() (rhi.zig:935-937) returns the proper ISSAOContext
• VULKAN_SSAO_VTABLE (rhi_vulkan.zig:854-856) wires to ssao_system.compute()

Scope is appropriate - removing ~3 declarations and their wiring is a single PR-sized change. The task is well-scoped and ready for implementation.

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