v0.16.3

sigil-ffi 0.16.3: never dlclose a library from the GC finalizer. Auto-unloading on collection was unsound (could dlclose a library whose C-side pointers, e.g. GLib main-context sources, are still live), crashing FFI-heavy dev-bundle programs. Matches ctypes/JNA/Guile; explicit c-library-close unchanged.