Introduce the first public C render-state surface for libghostty-vt. Before this change, the render-state path was only available in Zig, so C embedders had no direct way to create and update that cache. Add an opaque GhosttyRenderState type with new, update, and free entry points, then wire those symbols through the C API bridge and library exports. Keep the surface intentionally minimal for now so ownership and update behavior are established before adding read accessors.