0.18 round 2 (decrypt dedup): adopt DecryptingSectorSource at the two

existing call sites — sweep producer and DiscStream demux

Round 1 shipped the DecryptingSectorSource decorator
(libfreemkv/src/sector/decrypting.rs) but the existing decrypt
sites kept calling crate::decrypt::decrypt_sectors inline. This
commit migrates both:

- Disc::sweep (disc/mod.rs): producer wraps the input reader
  in DecryptingSectorSource::new(reader, keys) before the read loop.
  The inline decrypt_sectors call goes away — read_sectors yields
  plaintext directly.

- DiscStream (mux/disc.rs): constructor wraps the underlying
  Box<dyn SectorReader> in DecryptingSectorSource so the internal
  fill_extents / read path sees plaintext bytes. The DecryptKeys
  field stays on DiscStream for metadata-side use; it just no
  longer drives decryption.

Disc::patch carried the same inline decrypt step at three call
sites (main read, backtrack read, non-NOT_READY retry read). All
three migrated onto the same wrapping for a single audit surface.

Two small support changes carry the migration without touching
the round-1 decorator shape:
- sector/mod.rs gains specific SectorSource impls for
  &mut dyn SectorReader and Box<dyn SectorReader>, mirroring
  std's Read forwarding pattern. Generic blankets would conflict
  with the existing SectorReader → SectorSource blanket under the
  orphan rule (downstream could impl SectorReader for &mut U), so
  the impls are scoped to the dyn-trait shape we actually consume.
- sector/decrypting.rs gains DecryptingSectorSource::set_keys so
  DiscStream::set_raw() can flip the wrapped reader to a
  DecryptKeys::None pass-through without rebuilding the decorator
  (which would require moving the inner Box out from behind &mut self).

After this commit, grep `decrypt_sectors` in src/ shows the
function definition, its single use inside DecryptingSectorSource,
plus comments only. One audit surface for AACS / CSS / passthrough
correctness.

Behaviour-preserving: same plaintext bytes flow through; the only
difference is which type owns the decrypt step.

See (internal)/memory/0_18_redesign.md.

Single contributor: MattJackson.

src/sector/decrypting.rs

@@ -53,6 +53,16 @@ impl<S: SectorSource> DecryptingSectorSource<S> {
         self
     }
 
+    /// Replace the configured keys without unwrapping the decorator.
+    /// Used by `DiscStream::set_raw()` to flip from encrypted-disc
+    /// decryption to a pass-through after the inner reader is already
+    /// owned by the wrapper. For new construction prefer [`new`].
+    ///
+    /// [`new`]: Self::new
+    pub fn set_keys(&mut self, keys: DecryptKeys) {
+        self.keys = keys;
+    }
+
     /// Borrow the inner source. Useful for tests and for adapters
     /// that want to introspect the underlying drive / file without
     /// unwrapping the decorator.