ifcparse: skip flush+compact for read-only RocksDB on destruction

Read-only handles reject Flush/CompactRange, so the destructor's status
assertion always fired on shutdown when the streamer's sidecar was
opened with read_only=true. Track the flag and skip the write path; also
guard against a null db when the initial open failed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcparse/storage.h

@@ -390,6 +390,8 @@ namespace ifcopenshell {
             typedef rocksdb_map_adapter<inverse_attr_record, std::vector<uint32_t>> entities_by_ref_t;
             entities_by_ref_t byref_excl_;
 
+            bool read_only_ = false;
+
             // @todo naming
             rocks_db_file_storage(const std::string& path, ifcopenshell::file* owner_file, bool read_only = false);
             ~rocks_db_file_storage();