FEAT: Texture stylisation with Stable Diffusion

[nih23]

We already have shape-correct inputs (Y_shape from the renderer or NDS). This issue adds a texture-only stylization stage using Stable Diffusion 2.1 (img2img) with a style LoRA while ControlNet (Canny/Lineart) locks contours. The goal is to paint realistic Pecha paper tone, ink grain, edge roughness, and light bleed-through without moving strokes.