Test gaps: positioning, kitty sequences, multiple charts, detection #29

Not covered by tests yet:

• the kitty double-buffer sequence
• a layout move without a size change
• tmux and non-TTY detection
• raising errors in raw formats
• null x-values and tiny ranges in axes.ts
• the pixel dimensions chafa actually emits

Partly covered since #15: cursor save/restore and relative positioning, per-chart kitty IDs, and blanking on unmount and resize.