Expose selection endpoint ordering through the libghostty-vt C API so
embedders can safely normalize selections whose start and end refs may be
reversed. The new APIs report the current order and return a fresh
untracked selection with forward or reverse bounds.
Selection.Order now uses lib.Enum, matching the existing adjustment enum
pattern and keeping the C ABI enum generated from the same Zig source of
truth. The new functions are wired through the C API re-export and lib-vt
export paths, with coverage for mirrored rectangular selection ordering.
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