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/sector.rs

@@ -7,8 +7,47 @@
 use crate::error::Result;
 
 /// Read 2048-byte sectors from a disc or disc image.
-pub trait SectorReader {
+pub trait SectorReader: Send {
     /// Read `count` sectors starting at `lba` into `buf`.
     /// `buf` must be at least `count * 2048` bytes.
     fn read_sectors(&mut self, lba: u32, count: u16, buf: &mut [u8]) -> Result<usize>;
+
+    /// Total capacity in sectors, if known.
+    fn capacity(&self) -> u32 { 0 }
+}
+
+/// SectorReader backed by a file (ISO image).
+/// Seeks to lba * 2048, reads count * 2048 bytes.
+pub struct FileSectorReader {
+    file: std::io::BufReader<std::fs::File>,
+    capacity: u32,
+}
+
+impl FileSectorReader {
+    pub fn open(path: &str) -> std::io::Result<Self> {
+        let file = std::fs::File::open(path)?;
+        let len = file.metadata()?.len();
+        let capacity = (len / 2048) as u32;
+        Ok(Self {
+            file: std::io::BufReader::with_capacity(4 * 1024 * 1024, file),
+            capacity,
+        })
+    }
+}
+
+impl SectorReader for FileSectorReader {
+    fn read_sectors(&mut self, lba: u32, count: u16, buf: &mut [u8]) -> Result<usize> {
+        use std::io::{Read, Seek, SeekFrom};
+        let offset = lba as u64 * 2048;
+        let bytes = count as usize * 2048;
+        self.file.seek(SeekFrom::Start(offset))
+            .map_err(|e| crate::error::Error::IoError { source: e })?;
+        self.file.read_exact(&mut buf[..bytes])
+            .map_err(|e| crate::error::Error::IoError { source: e })?;
+        Ok(bytes)
+    }
+
+    fn capacity(&self) -> u32 {
+        self.capacity
+    }
 }