ifcparse: make deleting and creating instances work on RocksDB-backed files

file.remove() on a RocksDB-backed file segfaulted. Reducing it turned up
five gaps that each made editing such a file crash or silently do
nothing:

- process_deletion_inverse() decoded the v| inverse-record values as
  size_t while the serializer, register_inverse(), unregister_inverse()
  and instances_by_reference() use uint32_t, so std::find failed and
  vals.erase(end()) was undefined behaviour. It also took the DeleteRange
  end from an iterator that is invalid when the instance has no inverse
  records. Decode as uint32_t, remove every occurrence guarded on
  "found", and derive the range end from the prefix itself.
- attribute_value::size() ignored storage_model_, so every aggregate
  assignment on a RocksDB instance threw "Invalid variant index" from
  set_attribute_value(). Branch on the storage model like the sibling
  accessors and count the deserialized aggregate.
- rocks_db_file_storage::create() was a stub returning an empty handle,
  which anything creating an instance then dereferenced. Implement it
  after in_memory_file_storage::create().
- max_id_ is only initialised by the in-memory parse, so a RocksDB file
  would have handed out ids that overwrite existing instances.
  Implement the recalculate_id_counter() stub per backend and run it
  once before the first fresh_id() on RocksDB.
- byid_.erase() was a no-op: set_to_map_transformer::erase() and
  rocksdb_set_view::erase() were stubs. The deleted instance's attribute
  keys and cached handle survived, entity_names() still listed it and
  reopening the database threw. Erase deletes every key under the
  instance's prefix; the transformer forwards to it and takes an
  on-erase hook the storage uses to drop the cached handle.

root.remove_product on the first 200 products of a 61 MB model now
leaves the same surviving ids and inverse counts whether the file was
opened from SPF or converted to RocksDB.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HNrXDmR88wKPCYwGE21SyH

src/ifcparse/storage.h

@@ -678,6 +678,9 @@ namespace ifcopenshell {
             typedef std::map<uint32_t, shared_pointer_type> entity_by_iden_cache;
             entity_by_iden_cache instance_cache_, type_instance_cache_;
             std::mutex instance_cache_mutex_;
+            // Opening a database doesn't visit every instance, so the file's
+            // id counter is recalculated on the first create().
+            bool id_counter_recalculated_ = false;
 
             // @todo all these size_ts should probably be uint32_t for consistency with in-mem storage