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/aacs/variant.rs

@@ -201,14 +201,11 @@ pub fn walk_processing_key(
             // parse stops, no inner re-check needed.
             let u_mask_shift = uvs[5 * uvs_idx];
 
-            if u_mask_shift & 0xC0 != 0 {
-                break;
-            }
-            // 0x20..=0x3F (32..=63) pass the 0xC0 revoked-marker check but are
-            // out of range for a u32 shift. `wrapping_shl` would silently
-            // compute shift % 32 (e.g. 32 → no shift → 0xFFFF_FFFF), matching a
-            // wrong uv slot and deriving a wrong key. Disc-controlled byte:
-            // skip the slot instead.
+            // 0x20..=0x3F (32..=63) have their revoked-marker bits clear (so they
+            // pass the take_while above) but are out of range for a u32 shift.
+            // `wrapping_shl` would silently compute shift % 32 (e.g. 32 → no shift
+            // → 0xFFFF_FFFF), matching a wrong uv slot and deriving a wrong key.
+            // Disc-controlled byte: skip the slot instead.
             if u_mask_shift >= 32 {
                 continue;
             }