  Novel nanoscale probes are opening new venues for understanding
unconventional electronic and magnetic functionalities driven by multiscale
lattice complexity in doped high temperature superconducting perovskites. In
this work we focus on the multiscale texture at supramolecular level of atomic
oxygen interstitials (O-i) stripes in HgBa$_2$CuO$_{4+y}$ at optimal doping for
the highest superconducting critical temperature $T_C$=94K. We report
compelling evidence for the nematic phase of oxygen-interstitial O-i atomic
wires with fractal-like spatial distribution over multiple scales by using
scanning micro and nano X-ray-diffraction. The scale free distribution of O-i
atomic wires at optimum doping extending from micron scale down to nanoscale
has been associated with the intricate filamentary network of hole rich
metallic wires in the CuO$_2$ plane. The observed critical opalescence provides
evidence for the proximity to a critical point controlling the emergence of
high temperature superconductivity at optimum doping
