feat: add SixelDisplay backend for terminal graphics

- Implement pure Python Sixel encoder (no C dependency)
- Add SixelDisplay class to display.py with ANSI parsing
- Update controller._get_display() to handle sixel mode
- Add --display sixel CLI flag
- Add mise run-sixel task
- Update docs with display modes

mise.toml

@@ -33,6 +33,7 @@ run-poetry = "uv run mainline.py --poetry"
 run-firehose = "uv run mainline.py --firehose"
 
 run-websocket = { run = "uv run mainline.py --display websocket", depends = ["sync-all"] }
+run-sixel = { run = "uv run mainline.py --display sixel", depends = ["sync-all"] }
 run-both = { run = "uv run mainline.py --display both", depends = ["sync-all"] }
 run-client = { run = "mise run run-both & sleep 2 && $(open http://localhost:8766 2>/dev/null || xdg-open http://localhost:8766 2>/dev/null || echo 'Open http://localhost:8766 manually'); wait", depends = ["sync-all"] }