fix(cloud): spurious overlapping side-wall polygons on cloud shape

[Mandalorian-Wang]

Bug

The RenderIsoCloud function renders multiple overlapping data-face="side" polygons on the cloud node. In the test case the LLM node produced 8 side polygons all drawn at the same screen position with identical fill colour — no visual value, pure noise.

Root cause

The camera-facing visibility test (ny > 0) is applied per-edge on the complex 4-circle cloud outline. Because the outline has bumps with arcs that curve downward at multiple locations, the test finds several disconnected "visible" runs instead of the single contiguous lower silhouette. Each run is rendered as an independent polygon, all overlapping.

Fix (local commit 78558af on claude/adoring-feynman-6cklb1)

After collecting all visible runs, keep only the longest one — that is always the true lower silhouette. Discard the rest.

// Keep only the longest run — the true lower silhouette.
if len(runs) > 1 {
    best := 0
    for i, r := range runs {
        if len(r.idx) > len(runs[best].idx) {
            best = i
        }
    }
    runs = runs[best : best+1]
}

Side polygon count drops from 8 → 1. Golden file updated, all tests pass.

Reproduction

Any cloud node with non-trivial height (h > 10) will exhibit this — visible in data-face="side" count inside the rendered SVG fragment.