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/mux/ts.rs

@@ -167,6 +167,10 @@ impl PesAssembler {
                 self.buffer.clear();
                 self.active = false;
                 self.header_remaining = 0;
+                // A dropped partial PES is a gap in the elementary stream — flag
+                // it so the NEXT completed PES carries a discontinuity, matching
+                // every other partial-drop path in this file (lines 395/479/504).
+                self.pending_discontinuity = true;
                 return;
             }
             self.buffer.extend_from_slice(data);