Use chunked ISO search instead of reading entire 1.2GB file into memory

os.ReadFile on the ISO was OOM-killing the container. Switch to 8MB
chunked reads with overlap to handle matches spanning chunk boundaries.

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

internal/image/isomod.go

@@ -170,13 +170,16 @@ func rewriteGrubConfig(original, answerURL string) string {
 	return strings.Join(collapsed, "\n")
 }
 
-// findAllOccurrences searches the entire ISO file for all locations where
-// the grub.cfg content appears — this catches both the ISO9660 filesystem
-// copy and any copy inside an embedded EFI boot partition image.
 func findAllOccurrences(isoPath string, content []byte) ([]int64, error) {
-	data, err := os.ReadFile(isoPath)
+	f, err := os.Open(isoPath)
 	if err != nil {
-		return nil, fmt.Errorf("read ISO: %w", err)
+		return nil, fmt.Errorf("open ISO: %w", err)
+	}
+	defer f.Close()
+
+	stat, err := f.Stat()
+	if err != nil {
+		return nil, err
 	}
 
 	needle := content
@@ -184,20 +187,49 @@ func findAllOccurrences(isoPath string, content []byte) ([]int64, error) {
 		needle = needle[:256]
 	}
 
+	const chunkSize = 8 * 1024 * 1024
+	overlap := len(needle) - 1
+	buf := make([]byte, chunkSize+overlap)
+
 	var offsets []int64
-	start := 0
+	filePos := int64(0)
+
+	for filePos < stat.Size() {
+		readStart := filePos
+		if filePos > 0 {
+			readStart -= int64(overlap)
+		}
+
+		n, err := f.ReadAt(buf, readStart)
+		if n == 0 {
+			break
+		}
+
+		searchFrom := 0
+		if filePos > 0 {
+			searchFrom = overlap
+		}
+
+		chunk := buf[:n]
+		pos := searchFrom
 		for {
-		idx := bytes.Index(data[start:], needle)
+			idx := bytes.Index(chunk[pos:], needle)
 			if idx == -1 {
 				break
 			}
-		offsets = append(offsets, int64(start+idx))
+			offsets = append(offsets, readStart+int64(pos)+int64(idx))
-		start += idx + 1
+			pos += idx + 1
+		}
+
+		filePos += chunkSize
+		if err != nil {
+			break
+		}
 	}
 
 	if len(offsets) == 0 {
 		return nil, fmt.Errorf("grub.cfg content not found in ISO raw data (searched %d bytes with %d-byte needle)",
-			len(data), len(needle))
+			stat.Size(), len(needle))
 	}
 
 	return offsets, nil