/** Presentation only: exact opening, availability and event boundaries remain authoritative. */
export function calendarAxis(from: string | null, to: string | null) {
    const empty = { start: 0, end: 0, ticks: [] as string[], padding: [] as {
            starts_at: string;
            ends_at: string;
        }[] };
    if (!from || !to)
        return empty;
    const first = Date.parse(from), last = Date.parse(to);
    if (!Number.isFinite(first) || !Number.isFinite(last) || last <= first)
        return empty;
    const step = 30 * 60000;
    // Use local wall-time minutes from the API ISO offsets, including DST folds.
    const remainder = (iso: string) => Number(iso.slice(14, 16)) % 30 * 60000 + Number(iso.slice(17, 19)) * 1000;
    const start = first - remainder(from), end = last + (step - remainder(to)) % step;
    const ticks: string[] = [];
    for (let at = start; at < end; at += step)
        ticks.push(new Date(at).toISOString());
    const padding = [];
    if (start < first)
        padding.push({ starts_at: new Date(start).toISOString(), ends_at: from });
    if (last < end)
        padding.push({ starts_at: to, ends_at: new Date(end).toISOString() });
    return { start, end, ticks, padding };
}