binary_session reported native and artifact operations available whenever
their provider declared them, although these operations run on the active
target: in a closed session every call failed with target_unavailable, and
Mach-O-only operations stayed available with a ZIP or ELF target open.
Report target_required for every session-bound contract kind in a closed
session, and target_unsupported for Mach-O-only operations unless the open
target is a Mach-O executable. A host limitation is reported before target
selection, since opening a target cannot fix it; composed operations keep
deciding from their requirements.
Fixes#1166
Co-Authored-By: Claude Opus 5.5 <[email protected]>
