[async_scope] __future_opstate touches incomplete __future_state members when STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON

Requested by @ericniebler in #2271 — follow-up found while verifying that PR with -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON. Fails on pristine main, so it is separate from the fix in #2271.

Description

With -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON, building test/test_type_async_scope.cpp fails in include/exec/async_scope.hpp: __future_opstate accesses members of __future_state while that type is still incomplete. The debug machinery forces earlier instantiation of queries/members than a normal build does, exposing the incomplete type.

Reproduction

Configure/build the async_scope tests with the flag enabled (e.g. a TU including exec/async_scope.hpp compiled with -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=1). Reproduces on pristine main.

Suggested fix direction

Forward-declare what __future_opstate needs, or move the accessed members into a base class defined before __future_opstate, so the debug build's earlier instantiation point is valid.

Start with include/exec/async_scope.hpp and inspect how __future_opstate accesses the incomplete __future_state. Configure and build test/test_type_async_scope.cpp with STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=1 to reproduce the failure. Done means the async_scope test builds successfully on pristine main with the flag enabled.