feat: inline images in terminals (iTerm2 OSC 1337)

Images sent with the iTerm2 inline protocol (imgcat and friends) are
decoded (PNG, JPEG, GIF first frame) and drawn at the cursor, sized from
the width/height arguments in cells, pixels or percent (aspect kept,
never wider than the terminal); the cursor moves below so text
continues after them. They are anchored to the same stable line index
as the shell-integration marks, scroll with the text and are dropped
with the scrollback or on clear.

Remote data, so decoding is bounded (8192 px a side, 256 MiB
allocation), OSC bodies are capped at 16 MiB, inline=0 downloads are
ignored, and each terminal keeps at most 32 Mpx of images, oldest
first. Sixel and kitty graphics are not implemented.

docs/overview.md

@@ -73,6 +73,7 @@ Use Nexus Terminal when you need to:
 - Shift+PageUp/PageDown/Home/End and the mouse wheel scroll the scrollback; on the alternate screen the wheel sends arrow keys, so `tmux`, `screen` and `byobu` scroll naturally.
 - Context menu with Copy, Paste, Select All, Find, Send Snippet, Port Forwards, Split Right, Split Down, Export Output, Clear and Reset.
 - Export the whole scrollback as plain text.
+- Inline images in the iTerm2 format (`imgcat`, `wezterm imgcat`, `chafa -f iterm`…): PNG, JPEG and GIF, sized in cells, pixels or percent, scrolling with the text.
 - Shell integration (OSC 133): jump between prompts, copy the last command's output, and see each command's exit status and duration beside its prompt.
 - Status-bar latency for the active SSH session.