Major serialization pipeline refactor: all hot-path buffer and position management is now owned by BinarySerializationContext<TOutput>, with all write methods inlined for zero virtual dispatch. TOutput (now struct, IBinaryOutputBase) handles only cold-path buffer management (Initialize, Grow, GetTotalPosition). ArrayBinaryOutput and BufferWriterBinaryOutput are simplified to buffer managers. IBinaryOutput and BinaryOutputBase are removed. All serialization logic now uses context write methods (~130 call sites updated). This yields significant performance gains by eliminating virtual/interface calls on the serialization hot path.
