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/ifo.rs

@@ -9,7 +9,7 @@
 
 use crate::disc::{Codec, Resolution};
 use crate::error::{Error, Result};
-use crate::sector::SectorReader;
+use crate::sector::SectorSource;
 use crate::udf::UdfFs;
 
 // ── Public types ────────────────────────────────────────────────────────────
@@ -184,7 +184,7 @@ fn bcd_byte(b: u8) -> u32 {
 ///
 /// Reads the VMG (Video Manager) to discover title sets, then reads each
 /// VTS IFO to extract PGC chains, cell addresses, and stream attributes.
-pub fn parse_vmg(reader: &mut dyn SectorReader, udf: &UdfFs) -> Result<DvdInfo> {
+pub fn parse_vmg(reader: &mut dyn SectorSource, udf: &UdfFs) -> Result<DvdInfo> {
     let vmg_data = udf.read_file(reader, "/VIDEO_TS/VIDEO_TS.IFO")?;
 
     // Validate VMG magic
@@ -263,7 +263,7 @@ pub fn parse_vmg(reader: &mut dyn SectorReader, udf: &UdfFs) -> Result<DvdInfo>
 ///
 /// `titles_info` is a list of (chapter_count, vts_title_number) from TT_SRPT.
 fn parse_vts(
-    reader: &mut dyn SectorReader,
+    reader: &mut dyn SectorSource,
     udf: &UdfFs,
     vts_number: u8,
     titles_info: &[(u16, u8)],