Stamp the last frames that were still leaving without provenance

A real rip caught these. The warning added for a track placed from
timestamps under a seam plan fired once, on one subtitle track of a
23-clip title, eighteen minutes in — a display set that arrives as a lone
non-PCS segment is emitted straight through rather than accumulated, and
that path still built its frame with source: None.

Auditing the rest the same way found four more: flac, lpcm, mpegaudio and
the passthrough parser. All are one-PES-one-frame, so the packet's own
facts are the unit's facts.

Every Frame construction in every codec parser now carries a source,
checked by walking balanced braces rather than by eye — the earlier
count was taken with a regex that cannot see a block containing nested
braces, which is how these survived the first pass.

src/mux/codec/mpegaudio.rs

@@ -116,7 +116,7 @@ impl CodecParser for MpegAudioParser {
         vec![Frame {
             discontinuity: pes.discontinuity,
             coding: None,
-            source: None,
+            source: super::pesbuf::PesFacts::of(pes).source,
             pts_ns,
             keyframe: true,
             data: pes.data.clone(),