feat(sync): reconcile secondary Graph calendars

Read documented Graph v1 calendarView pages for secondary calendars and publish each finite window only after the complete bounded listing validates.

Add schema-v2 per-window event membership so moved-out occurrences are evicted without being mislabeled as remote tombstones, while overlapping windows retain shared instances.

Verified: 18/18 fresh normal, 18/18 -Werror, and 18/18 ASan/UBSan tests.

include/nocal/storage/sync_cache.hpp

@@ -76,6 +76,16 @@ struct PullPage {
     bool operator==(const PullPage&) const = default;
 };
 
+// A provider's complete view of one finite calendar window. Membership is
+// replaced only after the provider listing has completed successfully.
+struct CompleteWindow {
+    std::vector<CachedEvent> events;
+    std::vector<CachedEventInstance> instances;
+    SyncCheckpoint checkpoint;
+
+    bool operator==(const CompleteWindow&) const = default;
+};
+
 struct CacheSnapshot {
     std::vector<CachedAccount> accounts;
     std::vector<CachedCalendar> calendars;
@@ -88,7 +98,7 @@ struct CacheSnapshot {
 
 class SyncCache {
 public:
-    static constexpr int current_schema_version = 1;
+    static constexpr int current_schema_version = 2;
 
     explicit SyncCache(std::filesystem::path path);
     ~SyncCache();
@@ -112,6 +122,12 @@ public:
     // events retain their last raw payload and have no instances.
     void apply_pull_page(const PullPage& page);
 
+    // Atomically replaces membership for one complete, finite provider window.
+    // Returned events become live and have their instances replaced. Events
+    // omitted from the window lose cached instances only when no other complete
+    // window references them; omission is not treated as a remote tombstone.
+    void replace_complete_window(const CompleteWindow& window);
+
     [[nodiscard]] CacheSnapshot snapshot() const;
 
 private: