Add IOStream trait and stream-based I/O architecture

Introduce IOStream trait for uniform read/write across disc, file,
network, and null streams. Rename Title→DiscTitle, add stream URL
resolver, split old stream.rs into focused modules (m2ts, mkvstream,
network, disc, null, resolve, meta).

src/mux/null.rs

@@ -0,0 +1,43 @@
+//! NullStream — discards all data. Write-only. For benchmarking.
+
+use std::io::{self, Read, Write};
+use super::IOStream;
+use crate::disc::DiscTitle;
+
+/// Null stream — accepts writes, discards data. For benchmarking rip speed.
+pub struct NullStream {
+    disc_title: DiscTitle,
+    bytes_written: u64,
+}
+
+impl NullStream {
+    pub fn new() -> Self {
+        Self { disc_title: DiscTitle::empty(), bytes_written: 0 }
+    }
+
+    pub fn meta(mut self, dt: &DiscTitle) -> Self {
+        self.disc_title = dt.clone();
+        self
+    }
+
+    pub fn bytes_written(&self) -> u64 { self.bytes_written }
+}
+
+impl IOStream for NullStream {
+    fn info(&self) -> &DiscTitle { &self.disc_title }
+    fn finish(&mut self) -> io::Result<()> { Ok(()) }
+}
+
+impl Write for NullStream {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.bytes_written += buf.len() as u64;
+        Ok(buf.len())
+    }
+    fn flush(&mut self) -> io::Result<()> { Ok(()) }
+}
+
+impl Read for NullStream {
+    fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
+        Err(io::Error::new(io::ErrorKind::Unsupported, "null stream is write-only"))
+    }
+}