Accept F/D through the existing buffer packing implementation and restore
its complex round-trip test. Fix exact __complex__ results warning as
subclasses so valid conversions also work with warnings treated as errors.
Codex assisted with implementation, regression coverage, and review.
Tested: CPython 3.14.7 comparison; struct/memoryview/complex/cmath; workspace and C-API tests; Clippy; pytest; prek
Assisted-by: Codex:gpt-6-astra
