Fix read-fault misclassification, DTS AMODE channel table, and untestable guards

- resolve_fmts_key_map: distinguish a genuinely-not-FMTS disc from a
  transient live-drive read fault. read_filesystem now returns the new
  Error::UdfNotFilesystem for a deterministic tag/format mismatch (no AVDP,
  no partition descriptor, no FSD); resolve maps only UdfNotFilesystem (fs)
  and UdfNotFound (.tbl absent) to Ok(None), and PROPAGATES DiscRead / other
  I/O faults so a marginal AACS 2.1 disc fails loud instead of silently
  dropping forensic content under a base-Unit-Key-only map.

- DTS_AMODE_CH (mp4/audio.rs): extend 10→16 entries
  {1,2,2,2,2,3,3,4,4,5,6,6,6,7,8,8} (the spec per-AMODE channel table / ETSI TS 102 114) so
  the spec-legal high AMODEs that now pass the decodability gate declare
  their true channelcount (AMODE 13→7, 14/15→8) instead of a truncated 6.

- session.rs resolve_keys "called before scan" guard is now testable:
  from_parts_for_test takes Option<Disc>; added a test that a disc-less
  session returns a clean DeviceNotReady Err rather than panicking.

- mp4/read.rs: a track with samples but a missing/malformed stts (mandatory
  per ISO/IEC 14496-12) is dropped rather than emitting all-zero timestamps,
  matching the existing stco/stsc guards; all-tracks-dropped → Mp4Invalid.

- Remove the inert MuxInput::Iso.key_map field (the Iso path re-derives its
  map inside build_iso_pipeline); the live path keeps Live.key_map.

All four fixes are mutation-verified.

src/udf.rs

@@ -731,11 +731,9 @@ pub fn read_filesystem(reader: &mut dyn SectorSource) -> Result<UdfFs> {
 
     let tag_id = u16::from_le_bytes([avdp[0], avdp[1]]);
     if tag_id != 2 {
-        return Err(Error::DiscRead {
-            sector: 256,
-            status: None,
-            sense: None,
-        });
+        // Sector 256 read fine but carries no Anchor Volume Descriptor Pointer:
+        // this is deterministically not a UDF disc, not a transient read fault.
+        return Err(Error::UdfNotFilesystem);
     }
 
     // Main VDS extent location: bytes [16:20] = LBA, [20:24] = length
@@ -776,11 +774,8 @@ pub fn read_filesystem(reader: &mut dyn SectorSource) -> Result<UdfFs> {
     }
 
     if partition_start == 0 {
-        return Err(Error::DiscRead {
-            sector: 0,
-            status: None,
-            sense: None,
-        });
+        // No Partition Descriptor found in the VDS: structurally not a UDF disc.
+        return Err(Error::UdfNotFilesystem);
     }
 
     // Step 3: Parse partition maps from LVD to find metadata partition
@@ -871,11 +866,9 @@ pub fn read_filesystem(reader: &mut dyn SectorSource) -> Result<UdfFs> {
 
     let fsd_tag = u16::from_le_bytes([fsd[0], fsd[1]]);
     if fsd_tag != 256 {
-        return Err(Error::DiscRead {
-            sector: metadata_start as u64,
-            status: None,
-            sense: None,
-        });
+        // The metadata sector read fine but carries no File Set Descriptor:
+        // structurally not a UDF disc, not a transient read fault.
+        return Err(Error::UdfNotFilesystem);
     }
 
     // Root Directory ICB: long_ad at FSD offset 400