[idea] Sixel passthrough/rendering for Sixel-only outer terminals (foot, Mintty)

[sarniva]

idea / problem

Inline images don't render in Herdr panes when the outer terminal is Sixel-only (foot, Mintty, others). Same pane content renders on Kitty-graphics hosts (kitty, ghostty).

Repro (foot 1.28.0, Wayland, Omarchy + herdr 0.9.1 stable, terminal.kitty_graphics = true):

• Outside Herdr: opencode image attach previews correctly in foot (OpenTUI picks sixel), img2sixel renders.
• Inside Herdr pane on same foot: same image shows black box + [Image 1] (blocks fallback). img2sixel shows nothing.
• Same Herdr session from ghostty: images render.

Root cause per #3030 / #4330: Herdr only implements Kitty graphics passthrough (src/kitty_graphics.rs). Sixel emitted in panes is parsed/redrawn away, not forwarded. terminal.kitty_graphics explicitly covers "Kitty graphics-compatible outer terminals" only.

foot is the Omarchy default terminal, so Omarchy+Herdr users hit this for opencode attachments, yazi previews, plots, img2sixel.

requested change

Add Sixel support for panes, e.g.:

• parse Sixel (DECSIXEL) in pane VT, like Kitty placements today,
• re-emit/clip to Sixel-capable outer terminals (foot/Mintty/xterm/Konsole/WezTerm/VS Code/Windows Terminal),
• keep Kitty path untouched for Kitty hosts; pick per outer-terminal capability like OpenTUI does (kitty | sixel | blocks).

If full rendering is too big, even DCS passthrough of Sixel to a Sixel-capable host (à la tmux >= 3.4) would unblock img2sixel/static previews.

why you want this

• Sixel is the closest to a universal image protocol (foot, xterm, Konsole, Mintty, WezTerm, Windows Terminal, VS Code all support it; kitty/Ghostty are Kitty-only, Alacritty is neither).
• Without it, Sixel-only terminal users must leave Herdr or switch emulators to see any inline image.
• References: Sixel graphics emitted by  img2sixel  are not rendered in Herdr panes #3030 (Mintty/img2sixel, closed as feature-req), foot outer: inline images (sixel) not rendered in Herdr panes — Kitty-only passthrough #4330 (foot/opencode, closed as duplicate of Sixel graphics emitted by  img2sixel  are not rendered in Herdr panes #3030).

Environment: herdr 0.9.1 stable, foot 1.28.0, Arch/Omarchy Wayland, herdr server reload-config -> applied.