ifcparse: fixed-width hex ids in RocksDB keys, batched instance erasure

Review: recalculate_id_counter() scanned every i|<id>|_ record because
ids were written as decimal text, which doesn't sort numerically. Make
every numeric key segment fixed-width 16-digit lowercase hex, produced
and parsed by key_to_string()/key_from_string() in rocksdb_map_adapter.h,
with named builders (rocksdb_key::attribute, header_attribute,
type_record, inverse, inverse_prefix, type_list, upper_bound) that the
storage, entity_instance_data.cpp, read_schema() and the serializer use
instead of assembling "i|" + std::to_string(id) + ... by hand. Keys now
sort by id, an instance's records are contiguous in id order, and the
largest id is the last key under i|, which recalculate_id_counter()
seeks to. The two dormant to_string_fixed_width() helpers are gone.

This changes the on-disk layout; databases converted before this commit
have to be re-converted.

Also review: instance_cache_ eviction went through an on-erase hook on
set_to_map_transformer, a std::function call under a mutex per deleted
instance. Drop the hook; file::remove_entity() and unbatch() call
file::erase_instances_(ids), which on RocksDB is
rocks_db_file_storage::erase_instances(): one WriteBatch of DeleteRanges
and one lock for the whole batch.

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

src/ifcparse/storage.h

@@ -682,6 +682,10 @@ namespace ifcopenshell {
             // id counter is recalculated on the first create().
             bool id_counter_recalculated_ = false;
 
+            // Deletes every key of the given instances and drops their cached
+            // handles, in one write and under one lock.
+            void erase_instances(const std::vector<uint32_t>& ids);
+
             // @todo all these size_ts should probably be uint32_t for consistency with in-mem storage
 
             // lookup id->identity