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>

scripts/run-mpv.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Play the disc's feature straight from the ISO (or a BDMV folder) with mpv.
+#
+# mpv links libbluray and plays Blu-ray *titles/playlists* directly. It does NOT
+# run BD-J -- there is no menu, no Xlet, no Java involved. This is purely "open
+# the disc and watch the movie", and unlike the VLC/BD-J path it Just Works with
+# no JVM or cache setup. Use scripts/run-vlc.sh if you want the BD-J menu.
+#
+# Pass a path to override the default (an .iso image or a BDMV folder).
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+TARGET="${1:-$ROOT_DIR/build/adjustments.iso}"
+
+if [[ ! -e "$TARGET" ]]; then
+  echo "Missing disc target: $TARGET" >&2
+  echo "Run: make" >&2
+  exit 1
+fi
+
+if [[ -d "$TARGET" ]]; then
+  DEVICE="$(cd "$TARGET" && pwd)"
+else
+  DEVICE="$(cd "$(dirname "$TARGET")" && pwd)/$(basename "$TARGET")"
+fi
+
+# bd://longest plays the main title; --bluray-device points at the image/folder.
+exec mpv --bluray-device="$DEVICE" "bd://longest"