Cold breathes a crystal up a dark windowpane — drifting vapor that freezes where it touches, branching as real frost branches. No two panes are the same. Tune the cold, and see if you can make it live.
coverage —
branching (dim) —
reach (span) —
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Whether a pane is alive is measured, not admired: a lively frost is branched (box-counting dimension in a real range) and reaches across the glass (its largest crystal spans the pane). A bare pane or a clump of thin spikes fails, visibly and numerically. The same engine is checked headless in frost-verify.mjs.