Flip aggregate arguments and returns from the pointer / register-size
approximation to true by-value passing through the host_env `call` entry
point, and carry the driving layouts as host_env `CTypeLayout`.

- base.rs: `StgInfo.ffi_field_types: Vec<FfiType>` becomes
  `field_layouts: Vec<CTypeLayout>`, still built incrementally from the base
  class so struct inheritance is reflected. `StgInfo::to_ffi_type()` is
  replaced by a `type_layout(ty, &stg, vm)` helper that reads a type's own
  layout (aggregates from `field_layouts`, arrays recurse into the element
  type, simple types from their `_type_` code); it takes the already-borrowed
  `StgInfo` so it never re-locks the type. `CArgValue` gains an
  `Aggregate { layout, bytes }` variant lowering to `CallArg::Aggregate`, and
  `struct_union_paramfunc` snapshots the instance bytes into it (tag 'V').
- structure.rs / union.rs: collect each field's `type_layout` into
  `field_layouts` and store it on the finalized `StgInfo`.
- function.rs: `convert_object` passes a struct/union argument by value
  (snapshot bytes + argtype layout); a `byref()` result is still a pointer.
  `RetSpec` gains `Aggregate(CTypeLayout)`; `compute_ret_spec` returns it for
  a struct/union restype; `ctypes_callproc` lowers it to `CallRet::Aggregate`.
  `convert_raw_result` now drops the restype `StgInfo` read guard before
  constructing the result instance: instance construction write-locks the
  type's `StgInfo` to finalize it, which otherwise self-deadlocks against the
  held read guard (latent since the register-size return path, unreachable
  until a struct was actually returned by value).
- _ctypes.rs: a 'V' cparam now reprs as `<cparam 'V' at 0x..>` (the
  object-address default), matching PyCArg_repr; the aggregate `CArgValue`
  routes through that arm.
- extra_tests/snippets/stdlib_ctypes_byvalue.py: div/imaxdiv struct returns
  (8- and 16-byte), inet_ntoa struct and union arguments, with and without
  argtypes; output matches CPython. Also sorts the imports in the sibling
  stdlib_ctypes_calls.py snippet (ruff isort).

`_pack_` / `_swappedbytes_` layouts stay approximate — `CTypeLayout` carries
no explicit field offsets, as the previous libffi field-type path did not
either. host_env is untouched, so both `ctypes.rs` copies stay byte-identical.

test_ctypes is unchanged at run=322 skipped=58.

Assisted-by: Claude