mux: StreamSelection primitive + apply sites for per-title stream selection

The demux pipeline is declaration-driven off DiscTitle.streams (build_demux_state,
DiscStream::new, and the MKV writer all key off that list), so 'which streams to
keep' is already a pipeline capability with no public knob. This adds the knob:

- mux/select.rs: StreamSelection { audio, subtitle: PidFilter::All | Only(Vec<u16>) }
  + apply(&mut DiscTitle): keep Video always, keep Audio/Subtitle whose PID the
  filter lists, prune the rest (and the parallel codec_privates in lockstep);
  error SelectionPidUnknown on a listed PID absent from the title (fail loud, not
  a silently-missing track). Pure; 6 unit tests. Re-exported at crate root.
- Error::SelectionPidUnknown (E6014).
- MuxOptions gains  (+ derives Default now) applied in mux_stream's
  Iso/Session arms before the highway/DiscStream builds demux state (and before
  probe_and_remap's DVD AC-3 PID rewrite). InputOptions gains  applied
  in input()'s iso arm right after the title-index bounds check.

PIDs not languages -- language->PID is caller/engine policy. Default All/All is a
no-op (apply gated on !is_all()), so the no-selection path is byte-identical:
nothing below the title-finalization line changes (ts/ps/demux_thread/
pipelined_stream/mkv/disc untouched). All 2488 lib tests pass on 1.86.

src/lib.rs

@@ -272,6 +272,7 @@ pub use mux::{Mp4FitReport, Mp4SkipReason, mp4_fit_report};
 // `SectorSource` to get plaintext sectors out.
 pub use mux::build_iso_pipeline;
 pub use mux::resolve_mux_key_map;
+pub use mux::select::{PidFilter, StreamSelection};
 pub use mux::{MuxEvents, MuxInput, MuxOptions, MuxOutcome, NoopEvents, mux_stream};
 pub use scsi::{DriveInfo, ScsiSense, ScsiTransport, SenseFamily, drive_has_disc, list_drives};
 pub use sector::{