fix: assorted correctness fixes and dead-code cleanup

- aacs/resolve: a media-keys-only provider missing the VID classifies as
  VidUnavailable, not NoMaterial (an MK derives the VUK once the VID
  arrives).
- disc/bluray: mark a clip seen only after its .clpi parses, so a
  transient parse failure on the first PlayItem cannot suppress the
  clip's extents for a later PlayItem referencing it that succeeds.
- disc/patch: log rather than swallow mapfile record/flush failures on a
  reverify downgrade, so a failed persist cannot silently mismark a bad
  unit good on resume.
- mux/ts: flag a discontinuity when a partial PES is dropped, matching
  the other partial-drop paths.
- mux/demux_thread: the no-demuxer branch forwards an empty batch for
  early consumer-disconnect detection instead of reading the whole disc.
- io/pipeline: correct the send-timing log (as_secs_f64, not as_micros
  printed as ms).
- aacs/derive, aacs/variant, disc/read_error, keysource: comment/doc
  accuracy. sector/prefetched, udf: remove dead fields/functions.
- mux/disc: assert unit-aligned read counts in the test.

src/sector/prefetched.rs

@@ -358,7 +358,6 @@ impl PrefetchedSectorSource {
         // and suppresses `self`'s own `Drop` (which would otherwise
         // double-`join`), leaving NO extra live endpoint behind. This
         // is the panic-free equivalent of the `Option::take` approach.
-        let total = self.total_sectors;
         let me = std::mem::ManuallyDrop::new(self);
         // SAFETY: `me` is `ManuallyDrop`, so none of these fields will
         // be dropped by `me`. Each `ptr::read` performs exactly one
@@ -367,7 +366,7 @@ impl PrefetchedSectorSource {
         let producer = unsafe { std::ptr::read(&me.producer) };
         let rx = unsafe { std::ptr::read(&me.rx) };
         let recycle = unsafe { std::ptr::read(&me.recycle_tx) };
-        (rx, recycle, PrefetchShell { producer, total })
+        (rx, recycle, PrefetchShell { producer })
     }
 }
 
@@ -376,8 +375,6 @@ impl PrefetchedSectorSource {
 /// producer, even though the channels have been peeled off.
 pub struct PrefetchShell {
     producer: Option<JoinHandle<()>>,
-    #[allow(dead_code)]
-    total: u32,
 }
 
 impl Drop for PrefetchShell {