feat(parse): add ImageRef type and OSC 9 marker parser

Introduces ImageRef struct and ParseImageOSC() to parse OSC 9 image
markers (\033]9;image:path=...;alt=...\007) emitted by filters. Supports
BEL and ESC-backslash terminators, tolerates markers embedded in larger
lines, and rejects malformed or path-less markers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/parse/imageref_test.go

@@ -0,0 +1,95 @@
+package parse
+
+import (
+	"testing"
+)
+
+func TestParseImageOSC_ValidFull(t *testing.T) {
+	line := []byte("\033]9;image:path=/tmp/aerc-xxx/img0.png;alt=product photo\007")
+	ref, ok := ParseImageOSC(line)
+	if !ok {
+		t.Fatal("expected successful parse")
+	}
+	if ref.Path != "/tmp/aerc-xxx/img0.png" {
+		t.Errorf("path = %q, want /tmp/aerc-xxx/img0.png", ref.Path)
+	}
+	if ref.Alt != "product photo" {
+		t.Errorf("alt = %q, want 'product photo'", ref.Alt)
+	}
+}
+
+func TestParseImageOSC_PathOnly(t *testing.T) {
+	line := []byte("\033]9;image:path=/tmp/img.jpg\007")
+	ref, ok := ParseImageOSC(line)
+	if !ok {
+		t.Fatal("expected successful parse")
+	}
+	if ref.Path != "/tmp/img.jpg" {
+		t.Errorf("path = %q, want /tmp/img.jpg", ref.Path)
+	}
+	if ref.Alt != "" {
+		t.Errorf("alt = %q, want empty", ref.Alt)
+	}
+}
+
+func TestParseImageOSC_EscBackslashTerminator(t *testing.T) {
+	line := []byte("\033]9;image:path=/tmp/img.png\033\\")
+	ref, ok := ParseImageOSC(line)
+	if !ok {
+		t.Fatal("expected successful parse with ESC\\ terminator")
+	}
+	if ref.Path != "/tmp/img.png" {
+		t.Errorf("path = %q", ref.Path)
+	}
+}
+
+func TestParseImageOSC_NoPath(t *testing.T) {
+	line := []byte("\033]9;image:alt=just alt text\007")
+	_, ok := ParseImageOSC(line)
+	if ok {
+		t.Error("expected failure when path is missing")
+	}
+}
+
+func TestParseImageOSC_NotOSC9(t *testing.T) {
+	line := []byte("Hello, this is plain text")
+	_, ok := ParseImageOSC(line)
+	if ok {
+		t.Error("expected failure on plain text")
+	}
+}
+
+func TestParseImageOSC_WrongOSCNumber(t *testing.T) {
+	line := []byte("\033]8;image:path=/tmp/img.png\007")
+	_, ok := ParseImageOSC(line)
+	if ok {
+		t.Error("expected failure on OSC 8 (not 9)")
+	}
+}
+
+func TestParseImageOSC_EmptyPath(t *testing.T) {
+	line := []byte("\033]9;image:path=\007")
+	_, ok := ParseImageOSC(line)
+	if ok {
+		t.Error("expected failure on empty path")
+	}
+}
+
+func TestParseImageOSC_MarkerEmbeddedInLine(t *testing.T) {
+	line := []byte("prefix text \033]9;image:path=/tmp/img.png\007 suffix")
+	ref, ok := ParseImageOSC(line)
+	if !ok {
+		t.Fatal("expected successful parse even with surrounding text")
+	}
+	if ref.Path != "/tmp/img.png" {
+		t.Errorf("path = %q", ref.Path)
+	}
+}
+
+func TestParseImageOSC_NoTerminator(t *testing.T) {
+	line := []byte("\033]9;image:path=/tmp/img.png")
+	_, ok := ParseImageOSC(line)
+	if ok {
+		t.Error("expected failure when no terminator present")
+	}
+}