io+mux: phase 3 — streaming sinks + sequential container muxers

SocketSink + UdpSocketSink (`src/io/sink/socket.rs`) — sequential-only
TCP/UDP write destinations. SocketSink wraps BufWriter<TcpStream> with
1 MiB capacity, tunes SO_SNDBUF on construction, calls shutdown(Write)
on finish(). UdpSocketSink emits one datagram per write — caller
packetizes. Both impl Write+Send and thus satisfy SequentialSink via
the Phase 2 blanket; neither impls Seek, so RandomAccessSink is
correctly inaccessible (compile error to mux MKV onto a socket).

New sequential container muxers in src/mux/:

  - hevc/ — raw HEVC Annex B elementary stream. Length-prefixed NALU
    → 00 00 00 01 NALU. hvcC parsing emits VPS/SPS/PPS once at stream
    head. Fully ships.

  - m2ts_mux/ — standard MPEG-TS (188-byte packets). Single program,
    HEVC video on PID 0x100, optional AC3/TrueHD audio on PID 0x101.
    PAT+PMT re-emitted every 250 packets; PCR stamped on video every
    40 packets. Hand-rolled, no new deps. Distinct from the existing
    BD-TS (192-byte) `mux::m2ts::M2tsStream` — that path stays as-is.

  - fmp4/ — fragmented MP4. STUB: ftyp + minimal moov skeleton with
    one HEVC video trak + mvex/trex. Media fragments (moof+mdat) are
    TODO for v0.22.0 — write_video accumulates frames into a pending
    buffer that finish() clears. Init segment is well-formed enough
    that init_segment_starts_with_ftyp_then_moov asserts the box
    chain.

17 new unit tests added (socket round-trip, HEVC Annex B conversion,
M2TS packet alignment + PAT/PMT cadence + per-PID CC, fMP4 box chain).
All 514 lib tests + 17 new = pass on Rust 1.86 (fmt + clippy + test
via (internal)/scripts/precommit.sh libfreemkv).

No new dependencies. No version bump. Don't-touch list clean.

src/mux/mod.rs

@@ -33,6 +33,17 @@ pub(crate) mod m2ts;
 /// to round-trip codec_privates that don't fit inside the underlying format).
 /// Exposed for integration tests that exercise the wire format directly.
 pub mod meta;
+
+// ── Phase 3 sequential muxers ──────────────────────────────────────────────
+//
+// New container muxers that consume PES frames and write to a
+// `SequentialSink`. They are NOT refactors of the existing `MkvStream` /
+// `M2tsStream` (which round-trip via the legacy `Stream` trait + the
+// BD-TS framing); they're sequential-only and target the Phase 2 sink
+// split end-to-end.
+pub mod fmp4;
+pub mod hevc;
+pub mod m2ts_mux;
 pub(crate) mod mkv;
 pub(crate) mod mkvstream;
 pub(crate) mod network;