Halve the per-frame allocation and copy on the m2ts NAL video path

The NAL path called length_prefixed_to_annex_b, which allocates a whole-frame
Vec of its own, then copied the result into a second whole-frame Vec — two
full-frame allocations and two full-frame copies per video frame. The crate
already has append_length_prefixed_as_annex_b, which writes the conversion
straight into a destination buffer; it is the same code path with the
intermediate removed.

The destination is also sized once up front instead of starting from Vec::new(),
which re-grew from zero capacity inside every conversion.

On a UHD HEVC title muxed to m2ts:// — ~200k video frames averaging ~310 KB of
ES at 60 Mb/s — that removes roughly 62 GB of allocation and 62 GB of memcpy.

Behaviour is unchanged: the existing tsmux conversion tests, including the
non-NAL passthrough and Annex-B default pair added last round, all still pass.

A first attempt reused a persistent scratch buffer across frames, which does not
work: the buffer is handed out as Cow::Owned and so can never be returned. A
right-sized single allocation gets most of the win without restructuring the
function around the borrow.

src/mux/tsmux.rs

@@ -4,7 +4,7 @@
 //! packets. Each frame is wrapped in a PES header, split into TS packets,
 //! and prepended with the 4-byte TP_extra_header.
 
-use super::hevc::{avcc_to_annex_b, hvcc_to_annex_b, length_prefixed_to_annex_b};
+use super::hevc::{append_length_prefixed_as_annex_b, avcc_to_annex_b, hvcc_to_annex_b};
 use crate::disc::Codec;
 use std::io::{self, Write};
 
@@ -181,7 +181,10 @@ impl<W: Write> TsMuxer<W> {
         // unchanged, so borrow `data` directly rather than copying it; only
         // NAL video needs an owned Annex-B conversion buffer.
         let es_data: std::borrow::Cow<'_, [u8]> = if is_video && self.is_nal_video(track) {
-            let mut annex_b = Vec::new();
+            // Size the buffer once for the whole frame. `Vec::new()` re-grew from
+            // zero capacity on every frame, reallocating repeatedly inside a single
+            // ~310 KB conversion. The slack covers any prepended parameter sets.
+            let mut annex_b = Vec::with_capacity(data.len() + 1024);
             if keyframe && !self.params_written[track] {
                 if let Some(ref cp) = self.codec_privates[track] {
                     // avcC and hvcC are DIFFERENT box layouts; parsing one with
@@ -198,7 +201,12 @@ impl<W: Write> TsMuxer<W> {
                 }
                 self.params_written[track] = true;
             }
-            annex_b.extend_from_slice(&length_prefixed_to_annex_b(data));
+            // Write the conversion STRAIGHT into the destination.
+            // `length_prefixed_to_annex_b` allocates a whole-frame Vec of its own
+            // and we then copied it in, so every video frame cost two full-frame
+            // allocations and two full-frame copies. At ~200k frames averaging
+            // ~310 KB of ES on a UHD, that is ~124 GB of pointless memcpy.
+            append_length_prefixed_as_annex_b(&mut annex_b, data);
             std::borrow::Cow::Owned(annex_b)
         } else {
             if is_video {