Left: Period–density distribution of close-in Neptunian planets with precise masses and radii (4 ≤ R_p/R_⊕ ≤ 8.5). The red curve shows the tidal survival limit adopting η = 4.0. Vertical dashed lines mark the desert–ridge and ridge–savanna boundaries (Castro-González et al. 2024a), and horizontal dotted blue and red lines indicate the low- and high-density quantiles and their intersections with the tidal limit. The grey-shaded region highlights the low-density depletion, which effectively limits typical HEM outcomes to the ridge interval. Right: Normalised density distribution, defining the savanna as P_orb > 8 d.