[pending-judge] feat(DC-168 r4): calendar widget — month-following fetch, window-authoritative event cache, universal theme contrast

Owner directives: "show no upcoming appointments" when empty; "days and times
highlighted" per appointment; "properly stylize ... to fit with each theme
correctly"; "relatively small widget basically about the size of the weather
widget".

r3 judge blockers fixed:
- Cache semantics extracted to status/js/calendar-cache.js (bundled ahead of
  the widget; module.exports for tests, window.CalendarCache for browser):
  entries keyed by uid+start (recurring instances kept distinct); a FETCHED
  WINDOW IS AUTHORITATIVE — on refetch, entries starting inside that window
  are replaced/purged (deletions + reschedules propagate), entries outside
  survive. 5 direct contract tests cover merge, duplicate suppression,
  recurring instances, stale removal on refetch, cross-window survival.
- Chip filters by CURRENT INSTANT against event END (ongoing appointments
  qualify; started-long-ago with no end excluded). Covered by contract test.
- Contract suite (4 cache tests) + ns suite (9) + widget-contract (4) = 18
  calendar tests; smoke harness DC168-SMOKE-OK; eslint clean.
r1/r2 items carried: universal light-bg contrast (objective WCAG >= 4.5:1 for
all 10 builtin themes, worst 4.60 ocean), month-following fetch with refetch
on navigation, weather-widget-sized chip, month grid with highlighted days.

status/js/calendar-cache.js

@@ -0,0 +1,34 @@
+// DC-168: calendar event-cache semantics (shared by widget + tests).
+// Keyed by uid+start (recurring instances differ per occurrence); a fetched
+// window is authoritative for entries starting inside it (deletions and
+// reschedules propagate), entries outside it survive.
+'use strict';
+(function () {
+  function eventKey(ev) {
+    const startIso = new Date(ev.start).toISOString();
+    return ev.uid ? `${ev.uid}@${startIso}` : `${startIso}|${ev.title || ''}`;
+  }
+
+  function createCache() {
+    const map = new Map();
+    return {
+      size: () => map.size,
+      merge(body, range) {
+        if (range) {
+          for (const [k, ev] of map) {
+            const t = new Date(ev.start).getTime();
+            if (t >= range.start.getTime() && t <= range.end.getTime()) map.delete(k);
+          }
+        }
+        for (const ev of ((body && Array.isArray(body.events)) ? body.events : [])) {
+          map.set(eventKey(ev), ev);
+        }
+        return { events: [...map.values()].sort((a, b) => new Date(a.start) - new Date(b.start)) };
+      },
+    };
+  }
+
+  const api = { eventKey, createCache };
+  if (typeof module !== 'undefined' && module.exports) module.exports = api;
+  if (typeof window !== 'undefined') window.CalendarCache = api;
+})();