Adds CompositeMaterial + composite.wgsl: combines the HDR scene with the
bloom pyramid output, applies ACES (Narkowicz) tone mapping, and writes
LDR to the window. Removes UpscaleMaterial + upscale.wgsl.
mirror_params now updates composite (bloom_strength, exposure) and
brightpass (threshold) uniforms each frame. resize_offscreen rebuilds
the full bloom pyramid targets on window resize.
