Function coverageBlendPlan

Decide the coverage-band cross-fade for a scalar metric — whatever the group's selector names, i.e. the viewport AREA fraction under 'screen-area' (what derived ladders stamp) or the normalised diagonal (projected bbox diagonal ÷ FILL_FACTOR·fittedAxisPx, where fittedAxisPx is min(viewport.width, viewport.height)) under the legacy 'coverage' — against a level's ascending per-child coverage_fraction thresholds (coarsest 0 → finest 0.5 whole-object / 1.0 partition-bound under 'screen-area'; finest 1 for a legacy whole-object ladder, up to SCREEN_FILL_DIAGONAL_RATIO / FILL_FACTOR for an explicitly authored or partition-bound one). Everything here is proportional to the inter-threshold gaps, so the band scales with the ladder either way.

Each inter-level boundary is the activation threshold of the finer level (thresholds[i+1]). The band half-width is PROPORTIONAL to the local inter-level spacing — Δ_i = fraction · min(gapBelow, gapAbove) — because the coverage_fraction thresholds are geometrically spaced either way (a derived 'screen-area' ladder halves EXACTLY per coarser level, whatever the compression factor K; a derived legacy 'coverage' one steps by √K, from the retired sqrt(N_i/N_finest) derivation), so a constant band would be a clean dissolve at the finest step yet many times wider than the whole step at the coarse end (perpetually-blended coarse levels, overlapping bands). Scaling to the smaller adjacent gap makes the dissolve feel the same fraction of a step at every level and guarantees no two bands overlap (for fraction ≤ 0.5).

When the metric is within ±Δ_i of the NEAREST such boundary, the two levels straddling it cross-fade: the finer level's weight is smoothstep(boundary − Δ_i, boundary + Δ_i, metric), so it rises 0→1 across the band and is 0.5 at the boundary. The blend is continuous through the hard selection flip (which happens at the boundary): the same (lo, hi) pair and a continuous hiWeight govern both sides.

Returns null when no boundary's band contains the metric (a single level suffices), when fraction <= 0 (cross-fade disabled → hard step), or when there are fewer than two levels. Direction-agnostic — zoom-in and zoom-out cross the same band symmetrically.