mux: track skipped bytes for accurate loss estimation

DiscStream skips a whole AACS unit (3 sectors = 6144 bytes) per
read-error event, but only the skip-event count was exposed. Loss
estimates built from errors*2048 therefore undercounted AACS loss ~3x.

Add a lost_bytes field that accumulates the actual zero-filled byte
count at each skip, expose it via a new Stream::lost_bytes() accessor
(default 0; DiscStream and CountingStream override), so consumers can
scale lost-video time by real bytes lost rather than the event count.

Regression tests assert the AACS path records 6144 B/event (and
exceeds the errors*2048 undercount) while the align=1 path records
2048 B/event.

src/pes.rs

@@ -180,6 +180,17 @@ pub trait Stream: Send {
     fn errors(&self) -> u64 {
         0
     }
+
+    /// Cumulative bytes actually skipped (zero-filled) past read errors.
+    /// Distinct from [`errors`](Self::errors), which counts skip *events*:
+    /// a single AACS skip event covers a whole 6144-byte unit, so
+    /// `errors * 2048` understates real loss. Consumers estimating lost
+    /// video time must scale by this byte count, not the event count.
+    /// Default `0` for streams with no skip-on-error notion; `DiscStream`
+    /// overrides.
+    fn lost_bytes(&self) -> u64 {
+        0
+    }
 }
 
 /// Wraps any output stream and counts bytes written.
@@ -242,6 +253,10 @@ impl Stream for CountingStream {
     fn errors(&self) -> u64 {
         self.inner.errors()
     }
+
+    fn lost_bytes(&self) -> u64 {
+        self.inner.lost_bytes()
+    }
 }
 
 #[cfg(test)]