Add PES frame types and FileSectorReader — foundation for stream refactor

- pes.rs: PesFrame, InputStream, OutputStream traits
- sector.rs: SectorReader now public, added FileSectorReader (ISO = file)
- Foundation for unified DiscStream that handles both disc and ISO

src/pes.rs

@@ -0,0 +1,39 @@
+//! PES frame — the universal intermediate format.
+//!
+//! Every input stream produces PES frames. Every output stream consumes them.
+//! The pipeline just moves frames: input.next_frame() → output.write_frame().
+//!
+//! A PES frame is one unit of elementary stream data: a video frame,
+//! an audio frame, a subtitle packet. It has a track ID, timestamp,
+//! and the raw codec data.
+
+/// One frame of elementary stream data.
+#[derive(Debug, Clone)]
+pub struct PesFrame {
+    /// Track index (0-based, matches StreamInfo track order).
+    pub track: usize,
+    /// Presentation timestamp in nanoseconds.
+    pub pts: i64,
+    /// True if this is a keyframe (IDR for video).
+    pub keyframe: bool,
+    /// Raw elementary stream data (NAL units, audio samples, etc).
+    pub data: Vec<u8>,
+}
+
+/// Input stream — produces PES frames from any source.
+pub trait InputStream {
+    /// Get the next frame. Returns None at end of stream.
+    fn next_frame(&mut self) -> std::io::Result<Option<PesFrame>>;
+
+    /// Stream metadata (tracks, duration, etc).
+    fn info(&self) -> &crate::disc::DiscTitle;
+}
+
+/// Output stream — consumes PES frames to any destination.
+pub trait OutputStream {
+    /// Write one frame.
+    fn write_frame(&mut self, frame: &PesFrame) -> std::io::Result<()>;
+
+    /// Finalize (flush, write index, close).
+    fn finish(&mut self) -> std::io::Result<()>;
+}