v0.20.1: delete SectorReader, extract Disc::patch, doc/stub cleanup

WO-2 (delete SectorReader trait):
- The 0.18 trait split into SectorSource (read-only) and SectorSink
  (write-only) is final; the legacy SectorReader alias was a bridge.
- Renames every internal &mut dyn SectorReader (~25 sites) to
  &mut dyn SectorSource. The trait method capacity() becomes
  capacity_sectors() with a default of 0 (preserves SectorReader's
  default-0 behavior).
- Deletes the SectorReader trait, its blanket-to-Source bridge, and
  the FileSectorReader type alias. Adds explicit forwarding impls
  for Box<dyn SectorSource> and &mut dyn SectorSource so generic
  decorators like DecryptingSectorSource<S: SectorSource> compose.

WO-3a (extract Disc::patch):
- Moves Disc::patch (1230 lines) and bytes_bad_in_title from
  disc/mod.rs into disc/patch.rs as a split inherent impl. Zero
  behavior change — pure mechanical relocation. disc/mod.rs drops
  from 3,945 to 2,714 LOC.

WO-6 (partial):
- Deletes src/labels/png_filenames.rs — was a 72-LOC stub with
  detect() returning false, never wired into the PARSERS registry.

project docs doc drift fixes (audited 2026-05-13):
- JUMP_BASE_SECTORS: 256→1024 (64 MB base for UHD, not 8 MB)
- PASSN_DAMAGE_THRESHOLD_PCT: 12→6
- PASSN_SKIP_SECTORS_BASE: 64→32
- MAX_RANGE_SECS=180: replaced by proportional range_sectors × 25,
  capped at RANGE_BUDGET_CAP_SECS=1800.

src/mux/iso.rs

@@ -1,10 +1,10 @@
-//! ISO sector reader — file-backed SectorReader for Blu-ray ISO images.
+//! ISO sector reader — file-backed SectorSource for Blu-ray ISO images.
 //!
 //! An ISO is a flat image of 2048-byte sectors. Sector N starts at byte offset N * 2048.
 //! Used by DiscStream::open_iso() and Disc::scan_image().
 
 use crate::error::{Error, Result};
-use crate::sector::SectorReader;
+use crate::sector::SectorSource;
 use std::fs::File;
 use std::io::{Read, Seek, SeekFrom};
 use std::path::Path;
@@ -32,12 +32,12 @@ impl IsoSectorReader {
         Ok(Self { file, capacity })
     }
 
-    pub fn capacity(&self) -> u32 {
+    pub fn capacity_sectors(&self) -> u32 {
         self.capacity
     }
 }
 
-impl SectorReader for IsoSectorReader {
+impl SectorSource for IsoSectorReader {
     fn read_sectors(
         &mut self,
         lba: u32,
@@ -73,7 +73,7 @@ mod tests {
         std::fs::write(&dir, &data).unwrap();
 
         let mut reader = IsoSectorReader::open(dir.to_str().unwrap()).unwrap();
-        assert_eq!(reader.capacity(), 4);
+        assert_eq!(reader.capacity_sectors(), 4);
 
         let mut buf = [0u8; 2048];
         reader.read_sectors(0, 1, &mut buf, true).unwrap();
@@ -94,7 +94,7 @@ mod tests {
         std::fs::write(&dir, &data).unwrap();
 
         let reader = IsoSectorReader::open(dir.to_str().unwrap()).unwrap();
-        assert_eq!(reader.capacity(), 10);
+        assert_eq!(reader.capacity_sectors(), 10);
 
         std::fs::remove_file(&dir).ok();
     }