Phase 2 / Task 2: Move MaterialSystem ownership into render graph

[MichaelFisher1997]

Goal

Move material ownership out of RenderSystem and into the render-graph setup path.

Scope

• src/engine/graphics/render_system.zig
• src/engine/graphics/render_graph.zig

Work

• Rehome material-system setup where passes consume it
• Keep only the material interfaces required by passes
• Remove indirect access paths that exist only for pass execution

Acceptance criteria

• RenderSystem no longer stores material state only for pass execution
• Terrain and water material binding still works

Parallel-safe

Yes

[github-actions]

Based on my analysis, this is a refactoring task (Feature Request) with clear architectural implications.

Current Architecture:

• RenderSystem owns MaterialSystem (line 32, initialized at render_system.zig:193)
• SceneContext (render_graph.zig:66) receives *MaterialSystem
• Only 2 passes use it: OpaquePass (line 351) and WaterReflectionPass (line 521) via ctx.material_system.bindTerrainMaterial()

Root Cause: The indirect access pattern where RenderSystem owns MaterialSystem but only passes it through SceneContext to passes is unnecessary coupling.

Proposed Change: Initialize MaterialSystem during render-graph pass setup (before render_system.zig:214-229 where passes are added), and have passes store or receive the material interface they need directly rather than accessing via SceneContext.

Breaking down the scope:

1. Sub-task A: Move MaterialSystem.init() to before pass registration in RenderSystem.init
2. Sub-task B: Pass *MaterialSystem to OpaquePass and WaterReflectionPass at construction time (not via SceneContext execute)
3. Sub-task C: Remove material_system field from SceneContext once no passes use it
4. Sub-task D: Remove getMaterialSystem() from RenderSystem and the material_system: *MaterialSystem field

Impact: Low risk - only affects initialization order and pass interfaces. Terrain and water material binding will continue to work per acceptance criteria.

Implementation complexity: Moderate - requires tracking pass construction and initialization order since MaterialSystem depends on TextureAtlas which remains in RenderSystem.

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