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

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

Add res2k (2K-tuned) student pid_flux_2k.safetensors (sc-10056, epic 7840)

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

initial commit