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.go

@@ -0,0 +1,62 @@
+package parse
+
+import (
+	"bytes"
+	"strings"
+)
+
+// ImageRef represents an image referenced by a filter via OSC 9 marker.
+type ImageRef struct {
+	Index int    // Sequential position in the message
+	Path  string // Absolute path to image file on disk
+	Alt   string // Alt text fallback
+}
+
+var oscImagePrefix = []byte("\033]9;image:")
+
+// ParseImageOSC parses an OSC 9 image marker from a byte slice.
+// Format: \033]9;image:path=/tmp/foo.png;alt=text\007
+// The marker may appear anywhere in the line.
+// Returns the parsed reference and whether parsing succeeded.
+func ParseImageOSC(line []byte) (*ImageRef, bool) {
+	start := bytes.Index(line, oscImagePrefix)
+	if start == -1 {
+		return nil, false
+	}
+	rest := line[start+len(oscImagePrefix):]
+
+	// Find string terminator: BEL (\007) or ST (ESC \)
+	end := -1
+	for i := 0; i < len(rest); i++ {
+		if rest[i] == '\007' {
+			end = i
+			break
+		}
+		if rest[i] == '\033' && i+1 < len(rest) && rest[i+1] == '\\' {
+			end = i
+			break
+		}
+	}
+	if end == -1 {
+		return nil, false
+	}
+
+	params := string(rest[:end])
+	ref := &ImageRef{}
+	for _, pair := range strings.Split(params, ";") {
+		k, v, ok := strings.Cut(pair, "=")
+		if !ok {
+			continue
+		}
+		switch k {
+		case "path":
+			ref.Path = v
+		case "alt":
+			ref.Alt = v
+		}
+	}
+	if ref.Path == "" {
+		return nil, false
+	}
+	return ref, true
+}