feat(events): schedule, recurrence and the calendar (Phase 4)

The four closed recurrence shapes computed in the definition's own IANA zone,
a fourteen-day materialisation horizon with projections beyond it, series as a
managed thing, and the admin calendar that replaces the plugin this feature
exists to replace. An event now happens on its own.

No schema change: Phase 1 built every column this needed.

- events/recurrence.js is the ONE place an occurrence is computed, so the
  runner's expansion and the calendar's forecast cannot disagree. No date
  library added — Node ships the tzdata one would vendor, behind Intl.
- The runner's materialise leg is now two halves: expand, then sweep. The
  window starts at `now - grace`, so an occurrence nobody could have seen is
  never invented retroactively; the horizon is what makes the missed sweep
  mean anything for a recurrence.
- Publishing is the schedule switch and archiving turns it off, and publishing
  re-pins every occurrence that has not started.
- A projection is never drawn over an instant a run occupies, so a cancelled
  occurrence does not reappear as a forecast.

54 new tests, incl. the DST fixture set the plan asked for and three new
statements proved against a real MariaDB. Suite 1768/1711/56 skipped/1 fail
(pre-existing CRLF). Walked end to end on the local review stack.

Docs: RunicGateway/docs#PENDING

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/lib/eventAuthoring.js

@@ -141,7 +141,7 @@ export function formFromDefinition(event) {
     concurrencyKey: event?.concurrencyKey || '',
     graceSeconds: event?.graceSeconds ?? 900,
     timezone: event?.timezone || 'UTC',
-    scheduleKind: spec.schedule?.kind || 'manual',
+    ...scheduleFormFrom(spec.schedule),
     phases: (spec.phases || []).map((p) => ({
       key: p.key || '',
       label: p.label || '',
@@ -204,11 +204,136 @@ export function payloadFromForm(form) {
       concurrencyKey: form.concurrencyKey || null,
       graceSeconds: Number(form.graceSeconds),
       timezone: form.timezone,
-      spec: { schedule: { kind: form.scheduleKind || 'manual' }, phases },
+      spec: { schedule: scheduleFromForm(form), phases },
     },
   }
 }
 
+
+// ── The schedule (Phase 4) ─────────────────────────────────────────────────
+//
+// The four closed shapes of §E, mirrored so the form can render one and the
+// preview can describe it. `events/spec.js` and `events/recurrence.js` remain
+// the deciders — this is what makes the form a form rather than a text box, and
+// it is the whole reason the schedule is not a cron string: a closed set has a
+// dropdown, and an operator can proofread a dropdown.
+
+export const WEEKDAYS = [
+  'sunday',
+  'monday',
+  'tuesday',
+  'wednesday',
+  'thursday',
+  'friday',
+  'saturday',
+]
+
+export const SCHEDULE_KINDS = [
+  { value: 'manual', label: 'Started by hand' },
+  { value: 'once', label: 'Once, at a set time' },
+  { value: 'weekly', label: 'Weekly, on chosen days' },
+  { value: 'monthly', label: 'Monthly, on the nth weekday' },
+]
+
+// 1..4 and "last". There is no fifth: every month has a first through fourth of
+// every weekday, and "last" is what a month with five Fridays makes different
+// from "fourth" (org lead, 2026-09-02).
+export const MONTHLY_NTHS = [
+  { value: 1, label: 'First' },
+  { value: 2, label: 'Second' },
+  { value: 3, label: 'Third' },
+  { value: 4, label: 'Fourth' },
+  { value: -1, label: 'Last' },
+]
+
+const capitalise = (s) => String(s || '').charAt(0).toUpperCase() + String(s || '').slice(1)
+
+/**
+ * A schedule in words, in the event's own zone.
+ *
+ * The server says the same thing in `events/recurrence.js#describe`, and the two
+ * are allowed to differ on wording but not on meaning — this one is what an
+ * author reads while they are still typing, before anything has been saved.
+ */
+export function describeSchedule(schedule, timezone = 'UTC') {
+  if (!schedule || typeof schedule !== 'object') return 'No schedule'
+  const nth = MONTHLY_NTHS.find((n) => n.value === Number(schedule.nth))
+  switch (schedule.kind) {
+    case 'manual':
+      return 'Started by hand — nothing happens until an admin presses Start'
+    case 'once': {
+      if (!schedule.at) return 'Once — no date chosen yet'
+      return `Once, on ${String(schedule.at).replace('T', ' at ')} (${timezone})`
+    }
+    case 'weekly': {
+      const days = (schedule.days || []).map(capitalise)
+      if (!days.length || !schedule.time) return 'Weekly — choose days and a time'
+      const list =
+        days.length === 1
+          ? days[0]
+          : `${days.slice(0, -1).join(', ')} and ${days[days.length - 1]}`
+      return `Every ${list} at ${schedule.time} (${timezone})`
+    }
+    case 'monthly': {
+      if (!nth || !schedule.weekday || !schedule.time) {
+        return 'Monthly — choose a week, a weekday and a time'
+      }
+      return `The ${nth.label.toLowerCase()} ${capitalise(schedule.weekday)} of every month at ${schedule.time} (${timezone})`
+    }
+    default:
+      return 'No schedule'
+  }
+}
+
+/**
+ * The schedule half of the editor's working state.
+ *
+ * Every shape's fields are kept side by side rather than cleared when the kind
+ * changes, so an author who clicks Weekly, then Monthly, then back has not lost
+ * the days they picked. `scheduleFromForm` reads only the fields the chosen kind
+ * uses, which is what keeps the request body a clean single shape.
+ */
+export function scheduleFormFrom(schedule) {
+  const s = schedule || {}
+  return {
+    scheduleKind: s.kind || 'manual',
+    scheduleAt: s.kind === 'once' ? s.at || '' : '',
+    scheduleDays: s.kind === 'weekly' ? s.days || [] : [],
+    scheduleNth: s.kind === 'monthly' ? String(s.nth) : '1',
+    scheduleWeekday: s.kind === 'monthly' ? s.weekday || 'friday' : 'friday',
+    scheduleTime: s.kind === 'weekly' || s.kind === 'monthly' ? s.time || '20:00' : '20:00',
+  }
+}
+
+/** The schedule the form describes, as the spec object the server expects. */
+export function scheduleFromForm(form) {
+  switch (form.scheduleKind) {
+    case 'once':
+      return { kind: 'once', at: form.scheduleAt }
+    case 'weekly':
+      return { kind: 'weekly', days: form.scheduleDays || [], time: form.scheduleTime }
+    case 'monthly':
+      return {
+        kind: 'monthly',
+        nth: Number(form.scheduleNth),
+        weekday: form.scheduleWeekday,
+        time: form.scheduleTime,
+      }
+    default:
+      return { kind: 'manual' }
+  }
+}
+
+/**
+ * What a calendar entry is, and therefore what may be done with it.
+ *
+ * A `run` is a row: it has a console and somebody can cancel it. A `projected`
+ * entry is arithmetic the runner has not reached yet — there is nothing to open
+ * and nothing to stop, and an operator who treats one as a booking has been
+ * misled by the UI rather than by the server.
+ */
+export const isProjected = (entry) => entry?.kind === 'projected'
+
 /** An empty box is `{}`, not a parse error — a step may legitimately take none. */
 export function parseParams(text) {
   const raw = (text || '').trim()