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/encrypt.rs

@@ -2,7 +2,7 @@
 
 use super::*;
 use crate::error::{Error, Result};
-use crate::sector::SectorReader;
+use crate::sector::SectorSource;
 use crate::udf;
 
 /// Result of SCSI AACS handshake (ECDH authentication).
@@ -113,12 +113,12 @@ impl Disc {
 
     /// Resolve disc encryption — AACS 1.0, AACS 2.0, CSS, or none.
     ///
-    /// Reads AACS files from UDF (via SectorReader), resolves keys through
+    /// Reads AACS files from UDF (via SectorSource), resolves keys through
     /// whatever path works: KEYDB VUK lookup, media key derivation, processing
     /// keys, device keys. Uses handshake result (volume ID, bus key) if available.
     pub(super) fn resolve_encryption(
         udf_fs: &udf::UdfFs,
-        reader: &mut dyn SectorReader,
+        reader: &mut dyn SectorSource,
         keydb_path: &std::path::Path,
         handshake: Option<&HandshakeResult>,
     ) -> Result<AacsState> {