Add benchmark coverage for snapshot reader decoding and VT Writer formatting across empty, plain, Unicode, styled, and large terminal states. Previously each formatter call allocated a cgo writer handle and crossed into Go for every native fragment. Retain one handle per formatter and coalesce fragments in a binding-side buffer so steady-state writes are allocation-free and use bounded callbacks. Lazily install terminal effect userdata and co-allocate callback reader state with its snapshot decoder. This removes registry and wrapper allocations from decode paths while preserving on-demand effects for restored terminals.