Rendering: sRGB / linear color management

PBR lighting currently runs in gamma (display) space to stay consistent with the rest of the engine — albedo textures are sampled without sRGB→linear conversion and there is no output gamma/tonemap.

Scope

• Sample color textures (albedo, emissive) as sRGB (e.g. upload with an *_SRGB GPU format or convert in-shader); keep data maps (normal, metallic-roughness, occlusion) linear.
• Do lighting in linear space; apply tonemap (e.g. Reinhard/ACES) + gamma at output.
• Audit base_color / light-color authoring so existing scenes don't visually regress.
• Apply equivalently in the software renderer.

Acceptance

• Correct, consistent color across viewport and built game; no double-gamma; metals/roughness read physically.