docs: document PiD v1.5 flux2 checkpoint (_2606 retired) + fallback (sc-12144)

Add PiD v1.5 flux2 2kto4k student (sc-12144) — retires the _2606 color-drift workaround; coexists with v1.0

Add res2k (2K-tuned) student pid_flux2_2k.safetensors (sc-10056, epic 7840)

Re-host: PiD flux2 2kto4k (_2606) decoder (NVIDIA License / non-commercial) — converted from nvidia/PiD (sc-7852)

initial commit