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/labels/bdmt.rs

@@ -26,7 +26,7 @@
 // The module wiring (registry hook + public re-export) is added
 // separately. Until then the parse/detect entry points have no
 use super::xml;
-use crate::sector::SectorReader;
+use crate::sector::SectorSource;
 use crate::udf::UdfFs;
 use std::collections::BTreeMap;
 
@@ -60,7 +60,7 @@ pub fn detect(udf: &UdfFs) -> bool {
 /// Read every `bdmt_<lang>.xml` under `/BDMV/META/DL/` and return the
 /// aggregated [`DiscMetadata`]. Returns `None` if no titles could be
 /// extracted from any file.
-pub fn parse(reader: &mut dyn SectorReader, udf: &UdfFs) -> Option<DiscMetadata> {
+pub fn parse(reader: &mut dyn SectorSource, udf: &UdfFs) -> Option<DiscMetadata> {
     let dir = udf.find_dir("/BDMV/META/DL")?;
     let mut out = DiscMetadata::default();