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/disc.rs

@@ -0,0 +1,130 @@
+//! DiscStream — read BD-TS data from an optical disc drive.
+//!
+//! Read-only stream. Wraps DriveSession + Disc.
+//! Handles drive init, AACS decryption, and sector reading.
+
+use std::io::{self, Read, Write};
+use std::path::Path;
+use super::IOStream;
+use crate::disc::{DiscTitle, Disc};
+use crate::drive::DriveSession;
+use crate::error::Error;
+
+/// Options for opening a disc stream.
+pub struct DiscOptions {
+    /// Device path (e.g. "/dev/sg4"). None = auto-detect.
+    pub device: Option<String>,
+    /// KEYDB.cfg path. None = search standard locations.
+    pub keydb_path: Option<String>,
+    /// Which title to read (0-based). None = longest title.
+    pub title_index: Option<usize>,
+}
+
+impl Default for DiscOptions {
+    fn default() -> Self {
+        Self { device: None, keydb_path: None, title_index: None }
+    }
+}
+
+/// Optical disc stream. Read-only — yields decrypted BD-TS bytes.
+pub struct DiscStream {
+    disc_title: DiscTitle,
+    disc: Disc,
+    session: DriveSession,
+    title_index: usize,
+    // Read buffer: holds one batch from ContentReader
+    batch_buf: Vec<u8>,
+    batch_pos: usize,
+    started: bool,
+    eof: bool,
+}
+
+impl DiscStream {
+    /// Open the disc drive and scan disc metadata.
+    pub fn open(opts: DiscOptions) -> Result<Self, Error> {
+        let device = match opts.device {
+            Some(ref d) => crate::drive::resolve_device(d)?.0,
+            None => crate::drive::find_drive()
+                .ok_or_else(|| Error::DeviceNotFound { path: String::new() })?,
+        };
+
+        let mut session = DriveSession::open(Path::new(&device))?;
+        session.wait_ready()?;
+        let _ = session.init();
+        let _ = session.probe_disc();
+
+        let scan_opts = match opts.keydb_path {
+            Some(ref kp) => crate::disc::ScanOptions::with_keydb(kp),
+            None => crate::disc::ScanOptions::default(),
+        };
+        let disc = Disc::scan(&mut session, &scan_opts)?;
+
+        let title_index = opts.title_index.unwrap_or(0);
+        if title_index >= disc.titles.len() {
+            return Err(Error::DiscTitleRange { index: title_index, count: disc.titles.len() });
+        }
+        let disc_title = disc.titles[title_index].clone();
+
+        Ok(Self {
+            disc_title, disc, session, title_index,
+            batch_buf: Vec::new(), batch_pos: 0,
+            started: false, eof: false,
+        })
+    }
+
+    /// Get the full Disc (for listing all titles, etc.)
+    pub fn disc(&self) -> &Disc { &self.disc }
+}
+
+impl IOStream for DiscStream {
+    fn info(&self) -> &DiscTitle { &self.disc_title }
+    fn finish(&mut self) -> io::Result<()> { Ok(()) }
+}
+
+impl Read for DiscStream {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // Drain buffer first
+        if self.batch_pos < self.batch_buf.len() {
+            let n = (self.batch_buf.len() - self.batch_pos).min(buf.len());
+            buf[..n].copy_from_slice(&self.batch_buf[self.batch_pos..self.batch_pos + n]);
+            self.batch_pos += n;
+            return Ok(n);
+        }
+
+        if self.eof { return Ok(0); }
+
+        // Open reader on first call
+        if !self.started {
+            self.started = true;
+        }
+
+        // Read next batch via a temporary ContentReader
+        // ContentReader borrows session and disc, so we create it inline
+        let mut reader = self.disc.open_title(&mut self.session, self.title_index)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e.to_string()))?;
+
+        match reader.read_batch() {
+            Ok(Some(batch)) => {
+                let n = batch.len().min(buf.len());
+                buf[..n].copy_from_slice(&batch[..n]);
+                if batch.len() > n {
+                    self.batch_buf = batch.to_vec();
+                    self.batch_pos = n;
+                } else {
+                    self.batch_buf.clear();
+                    self.batch_pos = 0;
+                }
+                Ok(n)
+            }
+            Ok(None) => { self.eof = true; Ok(0) }
+            Err(e) => Err(io::Error::new(io::ErrorKind::Other, e.to_string())),
+        }
+    }
+}
+
+impl Write for DiscStream {
+    fn write(&mut self, _buf: &[u8]) -> io::Result<usize> {
+        Err(io::Error::new(io::ErrorKind::Unsupported, "disc is read-only"))
+    }
+    fn flush(&mut self) -> io::Result<()> { Ok(()) }
+}