Base: BD-J Adjustments Blu-ray scaffold

Vendored BD-J SDK, tsMuxeR + local VLC plugins, ffmpeg asset generation,
and a signed BD-J Xlet menu (org.homebrew.MyXlet) authored into a UDF 2.50 ISO.

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

docs/AUTHORING.md

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+# Adjustments BD-J Authoring Notes
+
+This project starts from the BD-J Linux SDK template disc metadata. The Xlet
+entry point is intentionally `org.homebrew.MyXlet` because
+`vendor/bdj-sdk/resources/AVCHD/BDMV/BDJO/00000.bdjo` launches that class.
+
+The build does three separate jobs:
+
+1. `ffmpeg` generates a temporary 720p23.976 MPEG-2 elementary stream and AC-3
+   audio.
+2. `tsMuxeR` authors those streams into Blu-ray playlist, clipinfo, and M2TS
+   files.
+3. The BD-J SDK signs `BDMV/JAR/00000.jar` and packages the final UDF 2.50 ISO.
+
+Useful commands:
+
+```sh
+make bootstrap
+make
+make inspect
+make run
+```
+
+Hidden-menu scaffold:
+
+- The visible menu has the main feature, two bonus-feature placeholders, and
+  settings.
+- Pressing `6`, `6`, `6` opens the hidden placeholder menu.
+- The generated main feature is playlist `00000`.
+
+For real assets later, keep the `00000` numbering until the BDJO/index metadata
+is regenerated or replaced with an authoring tool that owns those files too.