mux: make B1 concealment decode-clean on every gap shape

Closes the three residual holes where a concealed/lost gap could still let
a dangling-reference frame reach the muxer (degraded/undecryptable-disc
path only; clean rips are byte-identical and untouched). Root cause: the
discontinuity signal was reconstructed from the 4-bit continuity counter
and applied per-PES, both of which are lossy.

Three coordinated changes:

1. CC-INDEPENDENT marker. fill_null_ts_unit now tags its NULL packets with
   an adaptation-field discontinuity_indicator; the demuxer recognises a
   0x1FFF packet carrying it as a concealed gap and forces a discontinuity
   on every tracked PID (the lost unit's PID is unknowable). This survives
   a loss that is an exact multiple of 16 packets (CC aliases to in-sequence
   — hole 3) and a loss at a PID's very start (no prior CC — hole 4); it
   also drops any open, potentially-truncated partial PES.

2. PUSI ATTRIBUTION. A gap landing on a PES boundary now flags the PES
   STARTING after it, not the one flushed at the boundary (hole 1) —
   stamping the pre-gap frame could arm-then-disarm the gate on a keyframe
   and admit the real post-gap inter frame.

3. PER-FRAME signal. codec::Frame gains `discontinuity`; each parser
   propagates it onto the first post-gap frame. MPEG-2 buffers whole GOPs
   asynchronously, so it associates the gap by ES OFFSET (like PTS/source),
   landing it on the exact post-gap picture mid-GOP (hole 2) — a per-PES
   flag stamped the previous picture. consume_ts (and the EOF flush drain)
   gate on frame.discontinuity.

Tests: CC-independent marker with in-sequence CC + leading-loss; PUSI
attribution flags the post-gap PES; MPEG-2 offset-mark stamps the post-gap
picture through GOP reorder, not the previous one. Existing B1 gate + EOF
tests still green (2270 lib tests).

src/mux/codec/pgs.rs

@@ -57,6 +57,7 @@ impl PgsParser {
         let (start_pts, data) = self.pending.take()?;
         let duration = end_pts_ns.saturating_sub(start_pts).max(0) as u64;
         Some(Frame {
+            discontinuity: false,
             coding: None,
             source: None,
             pts_ns: start_pts,
@@ -92,6 +93,7 @@ impl CodecParser for PgsParser {
                 .take()
                 .map(|(start_pts, data)| {
                     vec![Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -119,6 +121,7 @@ impl CodecParser for PgsParser {
                 let frame = match pts {
                     Some(end) => self.emit_pending(end),
                     None => self.pending.take().map(|(start_pts, data)| Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -142,6 +145,7 @@ impl CodecParser for PgsParser {
                 // Flush any prior pending undurated and skip storing this one.
                 None => {
                     out.extend(self.pending.take().map(|(start_pts, data)| Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -167,6 +171,7 @@ impl CodecParser for PgsParser {
                     // (A missing PTS falls through to the drop path below: a
                     // bitmap with no timing reference would land at 00:00:00.)
                     out.push(Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: pts.unwrap_or(0),
@@ -195,6 +200,7 @@ impl CodecParser for PgsParser {
         // the final on-screen subtitle (see the module doc).
         match self.pending.take() {
             Some((start_pts, data)) => vec![Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns: start_pts,