narrow leaked-internal pub surface to pub(crate)

Verified zero callers across all consumer crates AND libfreemkv integration tests:
MUX_APP, Disc::{aacs_disc_hash,encrypted_content_ranges,inject_unit_keys},
locate_ranges, mapfile::{MapEntry,entries}, diag::dump_mkv_track,
DiscStream::{errors,lost_bytes} (read via accessors). Removed the unused mux
DemuxSink/FviSink crate-root re-exports (constructed internally by output() via
the direct module path). Staged demux option variants marked allow(dead_code).

src/disc/mapfile.rs

@@ -77,7 +77,7 @@ impl SectorStatus {
 
 /// One contiguous range of bytes with a status.
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub struct MapEntry {
+pub(crate) struct MapEntry {
     pub pos: u64,
     pub size: u64,
     pub status: SectorStatus,
@@ -498,7 +498,7 @@ impl Mapfile {
 
     /// All map entries, sorted ascending by `pos` and (after load)
     /// guaranteed disjoint and non-overflowing.
-    pub fn entries(&self) -> &[MapEntry] {
+    pub(crate) fn entries(&self) -> &[MapEntry] {
         &self.entries
     }