feat: replace polling with channel-based notifications and default to erofs

Replace the 500ms polling loop in waitForImageReady() with a channel-based
pub/sub notification system on the image manager, reducing build-to-SSE lag.
Switch the default image format from ext4 to erofs (LZ4-compressed read-only
filesystem) for faster, smaller rootfs images. The VM init mounts erofs first
with an ext4 fallback for backward compatibility with legacy images.

Log which filesystem type (erofs or ext4) was actually mounted so operators
can verify erofs is being used and diagnose fallback scenarios.

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

Author: hiroTamada

lib/system/init/mount.go

@@ -108,11 +108,14 @@ func setupOverlay(log *Logger) error {
 	// Mount readonly rootfs from /dev/vda
 	// Try erofs first (new default), fall back to ext4 (legacy images)
 	if err := mount("/dev/vda", "/lower", "erofs", "ro"); err != nil {
+		log.Info("hypeman-init:overlay", "erofs mount failed, trying ext4: "+err.Error())
 		if err := mount("/dev/vda", "/lower", "ext4", "ro"); err != nil {
 			return fmt.Errorf("mount rootfs: %w", err)
 		}
+		log.Info("hypeman-init:overlay", "mounted rootfs from /dev/vda (ext4 fallback)")
+	} else {
+		log.Info("hypeman-init:overlay", "mounted rootfs from /dev/vda (erofs)")
 	}
-	log.Info("hypeman-init:overlay", "mounted rootfs from /dev/vda")
 
 	// Mount writable overlay disk from /dev/vdb
 	if err := mount("/dev/vdb", "/overlay", "ext4", ""); err != nil {