mux: FMTS read-plan on the inline live-drive path too (DiscStream::with_key_map)

The read_plan fix landed on the file-backed highway (build_iso_pipeline)
but the inline DiscStream (live single-pass) still passed the alternate
device-group half to the demux — a known FMTS bug on rip_mode="single".

DiscStream::with_key_map installs the proactive map and rewrites the
extent walk to the read plan, so the live single-pass path reads ONLY
our-phase units, exactly like the highway. DecryptingSectorSource gains
set_key_map for the already-constructed decorator. Non-forensic maps
return the extents unchanged, so every non-FMTS disc is byte-identical.

Test: with_key_map_reads_only_our_phase_units — a forensic Even segment
drops exactly its alternate units from the extent walk. Precommit green
on 1.86.

src/sector/decrypting.rs

@@ -175,6 +175,14 @@ impl<S: SectorSource> DecryptingSectorSource<S> {
         self
     }
 
+    /// `&mut` counterpart of [`with_key_map`](Self::with_key_map): install the
+    /// proactive map on an already-constructed source (the inline live-drive
+    /// [`DiscStream`](crate::mux::DiscStream) builds the decorator first, then
+    /// installs the map via its own `with_key_map`).
+    pub fn set_key_map(&mut self, map: Arc<crate::decrypt::AacsKeyMap>) {
+        self.key_map = Some(map);
+    }
+
     /// Restrict decrypt to the disc's encrypted-content extents
     /// (sorted/merged `(start_lba, sector_count)` — see
     /// [`Disc::encrypted_content_ranges`](crate::Disc::encrypted_content_ranges)).