feat: embedded VTIMEZONE parsing, round-trip serialization

include/nocal/storage/ics_store.hpp

@@ -27,6 +27,15 @@ private:
     friend class IcsStore;
 };
 
+// Embedded VTIMEZONE definition preserved for round-trip serialization.
+struct VTimezoneDefinition {
+    std::string tzid;
+    // Complete raw content between BEGIN:VTIMEZONE and END:VTIMEZONE (exclusive
+    // of the framing lines). Empty when the definition was not parsed from a
+    // source calendar.
+    std::string raw;
+};
+
 struct LoadResult {
     std::vector<Event> events;
     std::vector<std::string> warnings;
@@ -36,6 +45,9 @@ struct LoadResult {
     // Exact bytes that produced this result. Copies share immutable storage and
     // can be supplied to save/restore as an optimistic concurrency token.
     FileRevision revision;
+    // Embedded VTIMEZONE definitions from the source calendar, preserved for
+    // round-trip serialization. Populated only by load().
+    std::vector<VTimezoneDefinition> vtimezones;
 };
 
 class IcsStore {
@@ -49,9 +61,11 @@ public:
     // requests durable filesystem flushes. I/O failures before replacement are
     // reported as std::runtime_error. If expected is present, the destination
     // must still contain the exact bytes represented by that revision.
+    // Supply vtimezones to preserve embedded timezone definitions.
     static FileRevision save(
         const std::filesystem::path& path, std::span<const Event> events,
-        std::optional<FileRevision> expected = std::nullopt);
+        std::optional<FileRevision> expected = std::nullopt,
+        std::span<const VTimezoneDefinition> vtimezones = {});
 
     // Restores the exact bytes in <path>.bak without changing the backup.
     static FileRevision restore_backup(