Harden 3D MVC mux: robustness + tests (audit round 1)

Triage of a 10-lens code audit of the 3D branch. Fixes for real defects;
rejected three spec false-positives that matched the ISO/IEC 14496-15
§7.6.2 record verbatim.

Robustness / correctness:
- Never panic when a title's only video is the MVC dependent view: the
  base is now the first NON-dependent video, so a dependent-only title
  sets up no merge (muxed as an ordinary track) instead of hitting an
  `expect` on the skipped track slot.
- Drop a per-frame BlockAdditional (BlockAddID=2) when the track declared
  no mvcC mapping (dependent params not captured before the header) — a
  plain block keeps the file conforming instead of an orphaned add.
- A non-keyframe MVC base frame always carries a ReferenceBlock (fall back
  to a 0 offset in the pre-first-keyframe corner) so it is never mistaken
  for a seek point.
- Reference the last keyframe on the PRIMARY video track only, so a
  secondary video track's keyframe can't become a cross-track reference.
- dep_by_pts overflow: bound BEFORE inserting so the just-arrived
  dependent survives the drift-clear; count a displaced duplicate-PTS
  dependent as an orphan instead of losing it silently.

API / docs:
- Fold write_frame_with_additional into write_frame(..., Option<&[u8]>)
  per the "no foo_with_X" convention.
- Fix mvc_params doc (StereoMode is intentionally not emitted); remove a
  stale PAT/PMT comment describing an approach that was never taken.

Tests: MVCDecoderConfigurationRecord over-length guards; write_int minimal
two's-complement widths; BlockGroup/BlockAdditions/BlockAdditional +
ReferenceBlock emission; additional dropped without a mapping; h264 MVC
passthrough keeps param sets in-band; extract_mvc_params no-panic on
truncated/empty input; pairing window + dep-overflow edges; no-panic on a
dependent-only title.

src/mux/codec/h264.rs

@@ -674,6 +674,46 @@ mod tests {
         v
     }
 
+    #[test]
+    fn mvc_passthrough_keeps_param_sets_inband() {
+        // A dependent-view access unit: subset SPS (NAL 15) + PPS (NAL 8) +
+        // coded-slice-extension (NAL 20). No IDR (type 5), so keyframe stays
+        // false and there is no keyframe re-assertion.
+        let au = || {
+            let mut d = Vec::new();
+            d.extend_from_slice(&h264_nal(0x6F, &[0x80, 0x00, 0x33, 0xAA])); // subset SPS (15)
+            d.extend_from_slice(&h264_nal(0x68, &[0xCE, 0x01])); // PPS (8)
+            d.extend_from_slice(&h264_nal(0x74, &[0x11, 0x22])); // slice-ext (20)
+            d
+        };
+        let nal_types =
+            |f: &Frame| -> Vec<u8> { h264_nals_in(&f.data).iter().map(|n| n[0] & 0x1F).collect() };
+
+        // Normal parser strips the PPS from a non-keyframe AU (it is captured for
+        // the avcC and, without an IDR, never re-asserted in-band).
+        let mut normal = H264Parser::new();
+        let f = normal.parse(&make_pes(au(), Some(90000)));
+        assert_eq!(f.len(), 1);
+        assert!(
+            !nal_types(&f[0]).contains(&8),
+            "normal parser strips PPS from a non-keyframe AU: {:?}",
+            nal_types(&f[0])
+        );
+
+        // Passthrough keeps EVERY parameter set in-band, so each dependent frame
+        // is a self-contained access unit for a BlockAdditional.
+        let mut pt = H264Parser::new().with_mvc_passthrough(true);
+        let f = pt.parse(&make_pes(au(), Some(90000)));
+        assert_eq!(f.len(), 1);
+        let types = nal_types(&f[0]);
+        assert!(types.contains(&15), "subset SPS kept in-band: {types:?}");
+        assert!(
+            types.contains(&8),
+            "PPS kept in-band under passthrough: {types:?}"
+        );
+        assert!(types.contains(&20), "slice kept: {types:?}");
+    }
+
     #[test]
     fn h264_populates_measured_coding_type_and_source() {
         use super::super::coding::CodingType;