Play feature under the menu; add mpv path; simplify VLC launch

Root causes found this session (all player-side, disc is valid -- bd_info
validates it and mpv plays the ISO's feature directly):

- Plain 'vlc <iso>' can't run ANY BD-J: system Java is 26, and libbluray's BD-J
  needs a JVM where System.setSecurityManager() works (<=21). run-vlc.sh pins
  mise Temurin 8, which is why it gets a menu at all.
- The BD-J VFS-cache 'error creating /tmp/libbluray-bdj-cache' flood is
  libbluray's SecurityManager.canWrite() denying the cache root while an Xlet
  context is active. Fixed by pointing TMPDIR at LIBBLURAY_CACHE_ROOT (buda
  root), which canWrite() allows unconditionally.
- Menu->playback failed with 'blurayReleaseVout: subpicture channel exists':
  VLC botches the background->playlist transition for an idle BD-J menu. Now the
  Xlet starts the feature immediately and draws the menu on top, so VLC stays in
  normal playing mode; MAIN FEATURE just hides the menu, any key brings it back,
  and the feature loops.

Also: drop the redundant tools/vlc-plugins swap (system VLC already ships the
bluray plugin), and add scripts/run-mpv.sh to play the feature with no BD-J/JVM
at all. Vendored the SDK helloworld sample for reference.

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

vendor/bdj-sdk/samples/helloworld/Makefile

@@ -0,0 +1,80 @@
+#   Copyright (C) 2022 John Törnblom
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING. If not see
+# <http://www.gnu.org/licenses/>.
+
+
+DISC_LABEL := helloworld
+
+#
+# Host tools
+#
+MAKEFILE_DIR := $(dir $(realpath $(lastword $(MAKEFILE_LIST))))
+BDJSDK_HOME  ?= $(MAKEFILE_DIR)/../../
+BDSIGNER     := $(BDJSDK_HOME)/host/bin/bdsigner
+MAKEFS       := $(BDJSDK_HOME)/host/bin/makefs
+JAVA8_HOME    ?= $(BDJSDK_HOME)/host/jdk8
+JAVAC        := $(JAVA8_HOME)/bin/javac
+JAR          := $(JAVA8_HOME)/bin/jar
+
+export JAVA8_HOME
+
+
+#
+# Compilation artifacts
+#
+CLASSPATH     := $(BDJSDK_HOME)/target/lib/enhanced-stubs.zip
+SOURCES       := $(wildcard src/org/homebrew/*.java)
+JFLAGS        := -Xlint:-options -source 1.3 -target 1.3
+
+
+#
+# Disc files
+#
+TMPL_DIRS  := $(shell find $(BDJSDK_HOME)/resources/AVCHD/ -type d)
+TMPL_FILES := $(shell find $(BDJSDK_HOME)/resources/AVCHD/ -type f)
+
+DISC_DIRS  := $(patsubst $(BDJSDK_HOME)/resources/AVCHD%,discdir%,$(TMPL_DIRS)) \
+              discdir/BDMV/JAR
+DISC_FILES := $(patsubst $(BDJSDK_HOME)/resources/AVCHD%,discdir%,$(TMPL_FILES)) \
+              discdir/BDMV/JAR/00000.jar
+
+
+all: $(DISC_LABEL).iso
+
+
+discdir:
+	mkdir -p $(DISC_DIRS)
+
+
+discdir/BDMV/JAR/00000.jar: discdir $(SOURCES)
+	$(JAVAC) $(JFLAGS) -cp $(CLASSPATH) $(SOURCES)
+	$(JAR) cf $@ -C src/ .
+	$(BDSIGNER) -keystore $(BDJSDK_HOME)/resources/sig.ks $@
+
+
+discdir/%: discdir
+	cp $(BDJSDK_HOME)/resources/AVCHD/$* $@
+
+
+$(DISC_LABEL).iso: $(DISC_FILES)
+	$(MAKEFS) -m 16m -t udf -o T=bdre,v=2.50,L=$(DISC_LABEL) $@ discdir
+
+
+clean:
+	rm -rf META-INF $(DISC_LABEL).iso discdir src/org/homebrew/*.class
+
+
+test: $(DISC_LABEL).iso
+	vlc $<