Move mouse event encoding logic from Surface.zig into a new input/mouse_encode.zig file. The new file encapsulates event filtering (shouldReport), button code computation, viewport bounds checking, motion deduplication, and all five wire formats (X10, UTF-8, SGR, urxvt, SGR-pixels). This makes the encoding independently testable and adds unit tests covering each format and edge case. Additionally, Surface `mouseReport` can no longer fail, since the only failure mode is no buffer space which should be impossible. Updated the signature to remove the error set.