UInt64SourceUnsigned 64-bit integer type for GLib guint64 values.
Backed by int64 internally — values above Int64.max_int are stored as negative int64 bit patterns, which round-trip correctly through GTK/GLib C calls via Int64_val / caml_copy_int64.
The type is kept abstract to prevent callers from accidentally using Int64.compare or other signed operations, which give wrong results for values ≥ 2^63. Use compare and equal for comparisons.