auto cover style: use sextants when fontconfig has a font with the glyphs

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/ui/cover_test.go

@@ -223,7 +223,39 @@ func TestSextantCell(t *testing.T) {
 	}
 }
 
+func TestCoverStyleForFontCheck(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
+
+	sextantFontAvailable = func() bool { return true }
+	for _, tc := range []struct {
+		name string
+		env  map[string]string
+		want string
+	}{
+		{"unknown terminal with a suitable font", map[string]string{"TERM": "xterm-256color"}, "sextant"},
+		{"over ssh the local fonts are unknown", map[string]string{"TERM": "xterm-256color", "SSH_CONNECTION": "1 2 3 4"}, "quadrant"},
+		{"over ssh with a tty", map[string]string{"SSH_TTY": "/dev/pts/1"}, "quadrant"},
+	} {
+		if got := coverStyleFor("auto", env(tc.env)).name; got != tc.want {
+			t.Errorf("%s: %s, want %s", tc.name, got, tc.want)
+		}
+	}
+	// An explicit setting never asks fontconfig.
+	sextantFontAvailable = func() bool { t.Error("fontconfig consulted for an explicit style"); return true }
+	if coverStyleFor("quadrant", env(nil)).name != "quadrant" || coverStyleFor("sextant", env(nil)).name != "sextant" {
+		t.Error("explicit styles must win")
+	}
+	// No suitable font: quadrants.
+	sextantFontAvailable = func() bool { return false }
+	if got := coverStyleFor("auto", env(map[string]string{"TERM": "xterm-256color"})).name; got != "quadrant" {
+		t.Errorf("no font: %s", got)
+	}
+}
+
 func TestCoverStyleFor(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	sextantFontAvailable = func() bool { return false } // the table below is about the terminal checks
 	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
 	for _, tc := range []struct {
 		setting string