pipeline: one run context, KeyRing, Source, Read stage, block and tree sinks

One run context (halt, events, loss counters) serves every stage; keys
come from KeyEvidence into a KeyRing. open_source opens every input,
disc:// included, replacing MuxSource; PesSource/PesSink replace
pes::Stream; one Read stage replaces DiscStream and the byte
prefetcher. iso://, null:// and dir:// are sinks. Dead error variants
(E9009, E9063, E9082) and scaffolding removed.

tests/disc_tests.rs

@@ -412,7 +412,7 @@ fn first_batch_sectors(sector_count: u32, batch_sectors: u16) -> usize {
             sector_count,
         }],
         batch_sectors,
-        None,
+        &libfreemkv::Ctx::default(),
     )
     .expect("prefetch producer spawns");
     let mut buf = vec![0u8; batch_sectors as usize * SECTOR_SIZE];
@@ -519,7 +519,7 @@ fn batch_count_zero_remaining() {
             sector_count: 0,
         }],
         60,
-        None,
+        &libfreemkv::Ctx::default(),
     )
     .expect("prefetch producer spawns");
     let mut buf = vec![0u8; 60 * SECTOR_SIZE];