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/set_to_map_transformer.h

@@ -36,11 +36,17 @@ public:
 private:
     BaseSet* base_map_;
     Transform transform_;
+    std::function<void(const key_type&)> on_erase_;
 
 public:
     set_to_map_transformer(BaseSet* base_set, Transform transform)
         : base_map_(base_set), transform_(transform) {}
 
+    // on_erase runs after erase(key), whether or not the base set still
+    // held the key, so state derived from the set can be dropped.
+    set_to_map_transformer(BaseSet* base_set, Transform transform, std::function<void(const key_type&)> on_erase)
+        : base_map_(base_set), transform_(transform), on_erase_(std::move(on_erase)) {}
+
     class iterator {
     public:
         using base_iterator = typename BaseSet::iterator;
@@ -107,8 +113,10 @@ public:
     }
 
     size_t erase(const key_type& key) {
-        static_cast<void>(key);
-        // @todo
-        return 0;
+        const size_t erased = base_map_->erase(key);
+        if (on_erase_) {
+            on_erase_(key);
+        }
+        return erased;
     }
 };