Update the pinned ghostty source to pick up the new render-cell GRAPHEMES_UTF8 data getter and GhosttyBuffer type. AppendGraphemes no longer carries a binding-local implementation of grapheme UTF-8 encoding. It now calls the upstream getter through a small C-stack wrapper, which keeps the GhosttyBuffer out-parameter off the Go heap while letting libghostty own the grapheme extraction and encoding behavior. This preserves the allocation-avoiding Go API while reducing the binding-side code to a thin adapter over the upstream API.