- date.js: single source of truth for ISO date formatting/arithmetic
(toISODate, parseISODate, addDays, daysBetween, todayISO, nowCompactTimestamp)
- event-factory.js: createEvent/cloneEvent (structuredClone with JSON fallback)
- split.js: splitRecurringEvent pure function using intervalDays from recur.js
- recur.js: replaced local toISO() and inline Date math with date.js helpers
- organize.js: replaced inline date formatting and gap math with date.js helpers,
extracted buildGrid() helper and byDate comparator
ICAL.Component instances break when wrapped by Vue's reactive proxy
(the .jCal getter returns undefined through the proxy, causing
serializeICS to crash with 'Cannot read properties of undefined').
Storing vtz.jCal (plain arrays) instead makes meta fully serializable
for both reactivity and JSON snapshot cloning.
