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/disc/bluray.rs

@@ -3,13 +3,13 @@
 use super::*;
 use crate::clpi;
 use crate::mpls;
-use crate::sector::SectorReader;
+use crate::sector::SectorSource;
 use crate::udf;
 
 impl Disc {
     /// Scan Blu-ray titles from MPLS playlists.
     pub(super) fn scan_bluray_titles(
-        reader: &mut dyn SectorReader,
+        reader: &mut dyn SectorSource,
         udf_fs: &udf::UdfFs,
     ) -> Vec<DiscTitle> {
         let mut titles = Vec::new();
@@ -31,7 +31,7 @@ impl Disc {
     }
 
     pub(super) fn parse_playlist(
-        reader: &mut dyn SectorReader,
+        reader: &mut dyn SectorSource,
         udf_fs: &udf::UdfFs,
         filename: &str,
         data: &[u8],
@@ -199,7 +199,7 @@ impl Disc {
     /// Prefers English, falls back to first available language.
     /// Returns None if META directory is empty or XML has no usable title.
     pub(super) fn read_meta_title(
-        reader: &mut dyn SectorReader,
+        reader: &mut dyn SectorSource,
         udf_fs: &udf::UdfFs,
     ) -> Option<String> {
         let meta_dir = udf_fs.find_dir("/BDMV/META")?;