docs: document PiD v1.5 qwenimage checkpoint + fallback (sc-12144)

Add PiD v1.5 qwenimage 2kto4k student (sc-12144) — improved color/corners/faces; coexists with v1.0

Re-host: PiD qwenimage 2kto4k decoder (NVIDIA License / non-commercial) — converted from nvidia/PiD model_ema_bf16.pth (sc-7852)

initial commit