Statistical Substructure of ISP Blazar Variability: Evidence for a Distinct High-Variability Population in 4LAC-DR3 (v6: Three-Source Analysis with W Comae)

Version 6 of the C3 ISP blazar paper. Adds Swift XRT analysis of W Comae (ON 231, 5,956 photons, 66.2 hours), completing a three-source case study of C3 members. Key new results: W Comae Fvar=0.883 (highest of three analyzed sources); spectral softening trend ΔHR=−0.118 (direction consistent with ON 246 and 3C 66A, marginally non-significant p=0.072 due to photon statistics). Section 3.5 renamed to 3.6 (three-source comparison table). New Section 4.5: Detection-Limit Hypothesis — Fvar-photon threshold relation. Three-source Fvar sequence: WComae(0.88) > 3C66A(0.57) > ON246(0.31). OSF: https://osf.io/7b5t2