refactor: extract date.js, event-factory.js, split.js; use shared date helpers in recur/organize

- 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

src/lib/event-factory.js

@@ -0,0 +1,41 @@
+/**
+ * Event model factory and helpers.
+ *
+ * The EventModel is the plain-JS shape used throughout the app (see
+ * docs/superpowers/specs/... §4.2). This module centralizes creation
+ * and cloning so that useCalendar.js stays focused on state management.
+ */
+import { todayISO, nowCompactTimestamp } from './date.js'
+
+/**
+ * Create a new single-occurrence event with sensible defaults.
+ * @param {object} [overrides] - partial fields to override
+ * @returns {object} a fresh EventModel
+ */
+export function createEvent(overrides = {}) {
+  return {
+    uid: crypto.randomUUID(),
+    summary: '新日程',
+    location: '',
+    description: '',
+    dtstartDate: todayISO(),
+    dtstartTime: '09:00',
+    dtendTime: '10:00',
+    tzid: 'UTC',
+    rrule: null,
+    exdates: [],
+    _raw: { dtstamp: nowCompactTimestamp() },
+    ...overrides,
+  }
+}
+
+/**
+ * Deep-clone an event via structuredClone (available in modern browsers
+ * and Node 17+). Falls back to JSON round-trip for older environments.
+ */
+export function cloneEvent(ev) {
+  if (typeof structuredClone === 'function') {
+    return structuredClone(ev)
+  }
+  return JSON.parse(JSON.stringify(ev))
+}