Use Unicode placeholders for kitty graphics inside tmux

[ibehnam]

Summary

• When running inside tmux, plots render as full-screen overlays instead of staying inside the tmux pane. This is because the current tmux passthrough approach sends raw graphics escape sequences to the outer terminal, which renders images at absolute cursor positions without pane awareness.
• This PR implements the kitty graphics protocol Unicode placeholder method. Image data is transmitted via tmux passthrough (a=T,U=1), but display uses U+10EEEE placeholder characters with diacritical marks encoding row/column positions. The image ID is encoded in the foreground RGB color. tmux treats these as normal text, so images stay inside their panes, scroll correctly, and get clipped at pane boundaries.
• Both Ghostty and Kitty support Unicode placeholders natively.
• The existing direct display path (outside tmux) is unchanged.

Changes

• utils.py: Add get_char_cell_width() and num_required_cols() helpers (mirrors existing height/lines helpers)
• backend.py: Add display_kitty_unicode_placeholder() with two-phase approach:
  1. Transmit image data via tmux passthrough
  2. Output Unicode placeholder characters as regular text
• backend.py: Route to Unicode placeholders when TMUX env var is set

Prerequisites

Requires allow-passthrough in tmux config:

set -g allow-passthrough on

Test plan

• Plot renders inside the tmux pane (not as a full-screen overlay)
• Switching panes hides the plot correctly
• Scrolling moves the plot with terminal content
• Works in both Ghostty and Kitty
• Plot still works correctly outside tmux (direct display, unchanged)

[mil-ad]

Thanks for the PR Behnam jan. I'll take a look soon

[ibehnam]

You're welcome Milad jan!

[mschubert]

I can confirm that tmux anchoring is broken and this fixes it for me