On direct disc reads (`disc://...`), `DiscStream` called `demuxer.feed(&self.read_buf[..bytes])`
instead of `feed_at(buf_base, ...)`. Without a running feed base offset, demuxed `PesPacket`s
had `source = None`, stripping byte provenance from downstream `PesFrame`s.
Under a `SeamPlan`, when `src_byte` is `None`, `SeamPlan::place` falls back to a timestamp
heuristic across clips. On multi-clip playlists where a subsequent clip starts at the same
base timestamp as the initial clip (`in_time = 600s`), B-frame PTS reorder dips (where
`raw_ns < last` and `|raw_ns - next_in| <= 250ms`) falsely triggered a `stepped_back` clip
transition on the first reordered frame. Because the subsequent clip's span was short (e.g. 1s),
all subsequent video frames across the rest of the title were rejected and dropped as outside
clip marks. Audio tracks (which have monotonic PTS without reordering) were unaffected.
Track `fed_bytes` in `DiscStream` and route through `feed_at` for both TS and PS demuxers,
forwarding `ps.source` to `PesPacket` on the PS path.
Test: `mux::disc::tests::stream_surface_tests::disc_stream_stamps_source_provenance`
