audit: clamp BD format fallback, running GOP byte counter, O(1) DTS marks

Round-8 findings from the 10-phase release audit:

- detect_disc_format's BDMV fallback passed detect_format's result
  through unchanged, so an SD bonus/menu title could tag a BD-tree disc as
  DVD (mis-sizing the ECC sweep) — violating its own "never below Blu-ray"
  invariant. Clamp anything but UHD up to Blu-ray.
- Track the MPEG-2 GOP byte total incrementally instead of re-summing the
  whole gop_buf on every pushed picture (was O(pictures²) on any MPEG-2
  disc, not just adversarial input).
- Back the DTS pts_marks deque with a VecDeque so the over-cap prune is an
  O(1) pop_front, not an O(n) Vec::remove(0).
- Add a test exercising parse_stream_id_extension's PTS/DTS skip branches
  (the real AU-opening 0xFD video PES path) — previously untested.

src/disc/mod.rs

@@ -2142,10 +2142,14 @@ impl Disc {
                     None => {}
                 }
             }
-            // Unencrypted / unreadable MKB: refine by resolution, default BD.
+            // Unencrypted / unreadable MKB: refine by resolution, but a BD-tree
+            // disc is never below Blu-ray — only UHD can promote it. detect_format
+            // is a general resolution classifier that can return Dvd for an SD
+            // bonus/menu title, which must NOT tag a BDMV disc as DVD (that
+            // mis-sizes the ECC-block sweep). Clamp anything but UHD up to BluRay.
             return match Self::detect_format(titles) {
-                DiscFormat::Unknown => DiscFormat::BluRay,
-                other => other,
+                DiscFormat::Uhd => DiscFormat::Uhd,
+                _ => DiscFormat::BluRay,
             };
         }
         if udf_fs.find_dir("/HVDVD_TS").is_some() {