Mux Blu-ray 3D (MVC) as a single MVC video track

Fold the MVC dependent (right-eye) view into the base H.264 track as a
per-frame BlockAdditional under an mvcC BlockAdditionMapping, so a 3D
title produces one MVC video track instead of two independent H.264
tracks.

- h264: MVC-passthrough parser mode keeps the dependent view's subset
  SPS/PPS in-band, so each emitted frame is a self-contained dependent
  access unit for a BlockAdditional
- resolve: route the dependent stream through the passthrough parser
- mkvstream: detect the dependent view, pair it to the base frame by
  PTS (bounded FIFO), attach it as a BlockAdditional (BlockAddID=2),
  and skip building its own track; build the mvcC
  MVCDecoderConfigurationRecord from the captured subset SPS/PPS and set
  it on the base track at activation
- mkv: emit the mvcC BlockAdditionMapping and BlockGroup/BlockAdditions,
  with a ReferenceBlock on non-keyframe base frames
- ebml: add BlockAdditions/BlockMore/BlockAdditional/BlockAddID/
  BlockAddIDValue/ReferenceBlock elements and a signed-int writer

Verified against a Blu-ray 3D ISO: ffprobe shows a single MVC track,
the mvcC mapping is present, ~144k BlockAdditionals carry the dependent
view (8.7 GB), and the base view decodes cleanly with no regression.
MVCDecoderConfigurationRecord follows ISO/IEC 14496-15 7.6.2; StereoMode
is intentionally omitted (no enum value describes MVC-in-BlockAdditional;
the mvcC mapping is the primary 3D signal per RFC 9559).

src/disc/bluray.rs

@@ -277,7 +277,7 @@ impl Disc {
                     streams.push(Stream::Video(VideoStream {
                         pid: dep_pid,
                         secondary: true,
-                        label: "MVC dependent view (3D right eye)".to_string(),
+                        label: crate::disc::MVC_DEPENDENT_LABEL.to_string(),
                         ..base
                     }));
                 }

src/disc/mod.rs

@@ -216,6 +216,22 @@ pub struct VideoStream {
     pub measured_cicp: Option<MeasuredCicp>,
 }
 
+/// Label marking a video stream as the Blu-ray 3D **MVC dependent (right-eye)
+/// view** — the paired substream of the AVC base view. Set by the BD scan
+/// (`bluray.rs`) and recognised by the mux path (`resolve.rs` builds its parser
+/// in param-set-preserving mode; `mkvstream.rs` folds it into the base track as
+/// per-frame `BlockAdditional`). Single source of truth for the contract.
+pub const MVC_DEPENDENT_LABEL: &str = "MVC dependent view (3D right eye)";
+
+impl VideoStream {
+    /// Whether this video stream is the MVC dependent (right-eye) view — the
+    /// 3D substream that the muxer merges into the base track as a per-frame
+    /// `BlockAdditional` rather than emitting as an independent track.
+    pub fn is_mvc_dependent(&self) -> bool {
+        self.label == MVC_DEPENDENT_LABEL
+    }
+}
+
 /// Measured CICP colour signalling read directly from a video elementary stream
 /// (ITU-T H.273). Preferred over the coarse [`ColorSpace`] enum when present.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]