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/imageref.go

@@ -7,7 +7,7 @@ import (
 
 // ImageRef represents an image referenced by a filter via OSC 9 marker.
 type ImageRef struct {
-	Index int    // Sequential position in the message
+	Index int    // Sequential position in the message; set by ExtractImages, not ParseImageOSC
 	Path  string // Absolute path to image file on disk
 	Alt   string // Alt text fallback
 }