labels: reader-backed detection, Criterion fix, menu-language fallback

- DetectFn now takes a SectorSource so a parser can inspect a jar's
  central directory in detect() instead of firing on "any BD-J jar".
  dbp/deluxe do the real com/<vendor>/ prefix check up front, so each
  claims only its own discs (foundational for scaling the registry).
- criterion: treat a stream-map value of 0 as unmapped and synthesize a
  real 1-based number, so a 0 can't shadow or collide with a genuine
  stream 1 (with regression tests).
- png_filenames: new Low-confidence, last-resort parser reading menu
  language from {title}_UHD01_{LANG}_Composite artwork; sits below the
  MPLS floor so a real framework parser always wins.
- vocab: add menu_lang() for 639-2/B to 639-2/T menu-token normalization.

src/labels/mpls_universal.rs

@@ -29,7 +29,7 @@ use crate::udf::UdfFs;
 
 /// True iff `/BDMV/PLAYLIST/` exists and contains at least one
 /// `.mpls` file. Cheap directory walk only — no sector reads.
-pub fn detect(udf: &UdfFs) -> bool {
+pub fn detect(_reader: &mut dyn SectorSource, udf: &UdfFs) -> bool {
     let Some(dir) = udf.find_dir("/BDMV/PLAYLIST") else {
         return false;
     };