Header F3d — the watermark hive, spacing refinements

Round 4. Ink is locked (F3d watermark opacities); what varies is the geometry — the problem found in round 3: the three same-size corner cells had uneven edge gaps (~2px diagonals vs ~4px horizontal), which reads as an accident precisely because the cells match. Options 1–3 fix it with a uniform gap at three tightnesses; option 4 goes the other way and makes the left cluster mixed-size like the echo (irregularity as intent); option 5 makes both sides formally regular.