feat(parse): add ExtractImages stream scanner for OSC 9 markers

Implements ExtractImages() which scans filter stdout line-by-line,
extracts valid OSC 9 image markers into ImageRef structs, and replaces
marker lines with null-delimited placeholders (\x00IMG:N\x00). Invalid
markers (missing path) pass through unchanged. Also clarifies the Index
field comment in ImageRef to indicate it is set by ExtractImages, not
ParseImageOSC.

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

lib/parse/extract_images.go

@@ -0,0 +1,41 @@
+package parse
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+
+	"git.sr.ht/~rjarry/aerc/lib/log"
+)
+
+// ExtractImages scans a reader for OSC 9 image markers, replaces them with
+// null-byte-delimited placeholders (\x00IMG:N\x00), and returns the cleaned
+// reader plus a slice of ImageRef structs.
+//
+// Lines containing valid image markers are replaced entirely with the
+// placeholder. Lines with invalid markers (e.g. missing path) pass through
+// unchanged.
+func ExtractImages(r io.Reader) (io.Reader, []ImageRef) {
+	var images []ImageRef
+	buf := bytes.NewBuffer(nil)
+	scanner := bufio.NewScanner(r)
+	scanner.Buffer(nil, 1024*1024*1024)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		if bytes.Contains(line, oscImagePrefix) {
+			if ref, ok := ParseImageOSC(line); ok {
+				ref.Index = len(images)
+				images = append(images, *ref)
+				fmt.Fprintf(buf, "\x00IMG:%d\x00\n", ref.Index)
+				continue
+			}
+		}
+		buf.Write(line)
+		buf.WriteByte('\n')
+	}
+	if err := scanner.Err(); err != nil {
+		log.Warnf("ExtractImages: scan error: %v", err)
+	}
+	return buf, images
+}