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/resolve.rs

@@ -106,8 +106,12 @@ pub fn resolve_keys_with_reason(
 pub(crate) fn classify_resolve_failure(ctx: &ResolveContext<'_>) -> ResolveFailure {
     let has_vid = *ctx.volume_id != [0u8; 16];
     let providers = super::provider::Providers(ctx.providers);
-    let has_derivation_material =
-        !providers.device_keys().is_empty() || !providers.processing_keys().is_empty();
+    // Media keys are also derivation material: with an MK you can derive the VUK
+    // once you have the VID, so a media-keys-only provider that is merely missing
+    // the VID is VidUnavailable, not NoMaterial.
+    let has_derivation_material = !providers.device_keys().is_empty()
+        || !providers.processing_keys().is_empty()
+        || !providers.media_keys().is_empty();
     if !has_vid && has_derivation_material {
         ResolveFailure::VidUnavailable
     } else {