Compare entity instances in one file by identity again

entity_instance.file is a property backed by a fresh SWIG wrapper on every
access, and ifcopenshell::file had no __eq__, so `self.file != other.file`
in entity_instance.__eq__ compared two throwaway wrappers and was always
true - even for an instance against itself. Every entity comparison
therefore took the deep get_info() branch, making distinct but structurally
identical instances compare equal and leaving the final `return False`
unreachable. Bonsai's TestAddRepresentationItemToShapeAspect showed this as
two separate IfcShapeAspects being treated as one, so the stale aspect was
never removed.

Restore the file_pointer() pair that was commented out on both
ifcopenshell::file and express::Base - IfcParseWrapper.i already described
it as the way to "trace file ownership of instances on the python side" -
and give file the __eq__/__hash__ it was missing. The express::Base one
needs $self->file() now that file_ lives on instance_data. This also
repairs rocksdb_lazy_instance.__eq__, which already called file_pointer().

EXPRESS `=` is value comparison and `:=:` is instance comparison, but
rule_compiler emits `==` for both (see the @todo on process_rel_op), and
derived attributes build their operands in the shared global file, so rules
compare same-file instances and need value semantics. Restore those for the
duration of rule execution with settings.compare_instances_by_value,
alongside the existing unpack_non_aggregate_inverses.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/ifcopenshell-python/ifcopenshell/ifcopenshell_wrapper.pyi

@@ -750,6 +750,7 @@ class entity_instance(entity_instance_mixin):
     def declaration(self) -> declaration: ...
     @property
     def file(self) -> ifcopenshell.file: ...
+    def file_pointer(self) -> int: ...
     def get_argument(self, *args: int | str) -> Any: ...
     def get_argument_index(self, a: str) -> int: ...
     def attribute_name(self, i: int) -> str: ...
@@ -836,6 +837,7 @@ class face:
     def print_impl(self, o, indent): ...
 
 class file(file_mixin):
+    def file_pointer(self) -> int: ...
     def fresh_id(self) -> int: ...
     def __init__(
         self,