Preview from BDMV folder so the BD-J menu Xlet actually loads

The disc structure, signed JAR, and video are all valid. The menu never
appeared because 'make run' opened the UDF .iso: libbluray's BD-J stack must
copy BDMV/JAR/00000.jar out of the image into /tmp/libbluray-bdj-cache before
the JVM can load it, and that write is refused by libbluray's BD-J
SecurityManager (usingUdf=true / no disc root). With no JAR mounted the Xlet
class isn't found (ClassNotFoundException: org.homebrew.MyXlet) and VLC falls
back to the raw playlist -> video, no menu.

Opening the build/disc BDMV folder instead lets libbluray read the JAR
directly, so the Xlet loads and the menu overlay runs. The ISO is still built
for burning; real players read the JAR from the disc filesystem directly.

- run-vlc.sh: default to build/disc, accept folder or .iso, add --bluray-menu
- Makefile: 'run' depends on the disc folder, not the ISO
- README: document the folder-vs-ISO libbluray caveat

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Makefile

@@ -102,8 +102,11 @@ inspect: $(ISO_FILE)
 	bd_info $(ISO_FILE)
 	bd_list_titles $(ISO_FILE) -a
 
-run: bootstrap $(ISO_FILE)
-	scripts/run-vlc.sh $(ISO_FILE)
+# Preview the menu straight from the BDMV folder. libbluray loads the BD-J Xlet
+# from a folder but not from a UDF .iso on this setup (see scripts/run-vlc.sh),
+# so `run` intentionally does not depend on the ISO.
+run: bootstrap $(JAR_FILE)
+	scripts/run-vlc.sh $(DISC_DIR)
 
 open: run