Ask the terminal before using Kitty graphics; no fontconfig-based sextants on macOS

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/probe_unix.go

@@ -0,0 +1,57 @@
+//go:build unix
+
+package ui
+
+import (
+	"bytes"
+	"os"
+	"time"
+
+	"golang.org/x/term"
+)
+
+// probeKittyGraphics asks the terminal whether it understands the Kitty
+// graphics protocol: a tiny query image (which a capable terminal answers with
+// "OK" and does not show) followed by a Primary Device Attributes request,
+// which every terminal answers. If the DA answer arrives without a graphics
+// answer before it, the protocol is not supported. Runs before the screen is
+// set up, on the controlling terminal.
+func probeKittyGraphics(timeout time.Duration) bool {
+	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
+	if err != nil {
+		return false
+	}
+	defer tty.Close()
+	fd := int(tty.Fd())
+	state, err := term.MakeRaw(fd)
+	if err != nil {
+		return false
+	}
+	defer term.Restore(fd, state)
+
+	const query = "\x1b_Gi=31,s=1,v=1,a=q,t=d,f=24;AAAA\x1b\\" + "\x1b[c"
+	if _, err := tty.WriteString(query); err != nil {
+		return false
+	}
+	// A terminal that does not know APC sequences may print the query; erase it.
+	defer tty.WriteString("\r\x1b[2K")
+
+	if err := tty.SetReadDeadline(time.Now().Add(timeout)); err != nil {
+		return false
+	}
+	var got []byte
+	buf := make([]byte, 256)
+	for {
+		n, err := tty.Read(buf)
+		got = append(got, buf[:n]...)
+		if bytes.Contains(got, []byte("_Gi=31;OK")) {
+			return true
+		}
+		if da := bytes.Index(got, []byte("\x1b[?")); da >= 0 && bytes.IndexByte(got[da:], 'c') >= 0 {
+			return false // device attributes answered, graphics query not
+		}
+		if err != nil {
+			return false // timeout or error: assume no support
+		}
+	}
+}