[R2-5][P2] Convert updateGlobalUniforms 13-param signature to GlobalUniforms struct

[MichaelFisher1997]

Part of #839 — [Audit][Round 2] A++ umbrella.
Phase: P2 · Finding: R2-5

Problem

IRenderStateContext.updateGlobalUniforms takes 13 positional parameters. Adding a single uniform (e.g. SSAO intensity, time-of-day weight) means editing the signature across every caller, the wrapper, the vtable field, and the backend implementation. This is textbook shotgun surgery and an AGENTS.md-style readability violation.

Evidence

modules/engine-rhi/src/rhi.zig:354 — wrapper signature (one of 4 sites):

pub fn updateGlobalUniforms(self: RenderContext, view_proj: Mat4, cam_pos: Vec3,
    sun_dir: Vec3, sun_color: Vec3, time: f32, fog_color: Vec3, fog_density: f32,
    fog_enabled: bool, sun_intensity: f32, ambient: f32, use_texture: bool,
    frame_params: FrameRenderParams) !void

4 mirrored call sites for the same 13 params:

• rhi.zig:354 — RenderContext.updateGlobalUniforms (wrapper)
• rhi.zig:355 — delegates to self.state.updateGlobalUniforms(...) (same 13 args)
• rhi.zig:596 — IRenderStateContext.VTable.updateGlobalUniforms (function pointer type)
• rhi.zig:615-616 — IRenderStateContext.updateGlobalUniforms (interface method)

Plus every backend implementation (rhi_vulkan.zig and the corresponding vulkan/rhi_render_state.zig) and every caller:

• engine-graphics/src/render_graph.zig (the dominant caller)
• src/game/app.zig:397
• modules/game-ui/src/screens/world.zig:397

Adding a uniform today = 4 signature edits in rhi.zig + 1 in backend + N in callers.

Fix

Introduce GlobalUniforms struct (mirrors the existing FrameRenderParams pattern at rhi_types.zig:298):

// modules/engine-rhi/src/rhi_types.zig
pub const GlobalUniforms = struct {
    view_proj: Mat4,
    cam_pos: Vec3,
    sun_dir: Vec3,
    sun_color: Vec3,
    time: f32,
    fog_color: Vec3,
    fog_density: f32,
    fog_enabled: bool,
    sun_intensity: f32,
    ambient: f32,
    use_texture: bool,
    // Future uniforms land here — no signature change required.
};

pub const FrameRenderParams = struct { ... }; // unchanged

New signatures:

// Wrapper
pub fn updateGlobalUniforms(self: RenderContext, uniforms: GlobalUniforms, frame_params: FrameRenderParams) !void

// VTable
updateGlobalUniforms: *const fn (ptr: *anyopaque, uniforms: GlobalUniforms, frame_params: FrameRenderParams) anyerror!void,

Update 4 sites in rhi.zig + 1 backend site + N callers. After this, adding a uniform = 1 struct field edit + N callers that opt in (others can use default .{ ... } literal).

Verification

• nix develop --command zig build test (includes shader validation — uniform layout must match shader uniform blocks)
• nix develop --command zig build test -- --test-filter "RenderGraph"
• Required: headless screenshot baseline before/after (-Dskip-present, low/medium presets) — visual parity must be bit-exact (uniform byte layout must not change)
• Grep verification: grep -rE "updateGlobalUniforms\(" modules/ src/ should show only 2-arg calls

Constraints

• Behavior-preserving. Shader uniform block layout must not change.
• Single PR — straightforward mechanical refactor.
• Conventional commit: refactor(engine-rhi): bundle updateGlobalUniforms args into GlobalUniforms struct
• While here, consider migrating the anyerror! return at rhi.zig:596 to RhiError! (small bonus).

Notes

• Pattern is already established by FrameRenderParams at rhi_types.zig:298 — this is a consistency fix, not a new idiom.
• Stretch: consider GlobalUniforms as extern struct if shader binding wants a stable byte layout (check current staging code first).

Tracking: #839