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/session.rs

@@ -381,9 +381,12 @@ impl DiscSession {
     /// The drive slot stays `None` (a `MuxInput::Session` mux never touches it —
     /// it reads through the staged `reader`); `device` carries a sentinel path so
     /// the driver's missing-reader error still has a name.
+    ///
+    /// `disc` is an `Option` so a test can construct a session that has NOT been
+    /// scanned (`None`) to exercise the `resolve_keys` "called before scan" guard.
     #[cfg(test)]
     pub(crate) fn from_parts_for_test(
-        disc: Disc,
+        disc: Option<Disc>,
         reader: Option<Box<dyn SectorSource>>,
         key_fetch: Option<KeyFetch>,
     ) -> DiscSession {
@@ -391,7 +394,7 @@ impl DiscSession {
             drive: None,
             device: "test://session".to_string(),
             spec: KeySpec::default(),
-            disc: Some(disc),
+            disc,
             reader,
             key_fetch,
         }
@@ -733,4 +736,22 @@ mod tests {
             "the disc is left untouched"
         );
     }
+
+    /// `resolve_keys` called before `scan` (disc slot still `None`) must return the
+    /// clean typed `DeviceNotReady` guard, never reach the `.expect("disc present
+    /// (checked above)")` below it and panic.
+    ///
+    /// Mutation: change the `if self.disc.is_none()` guard to `.expect()`/panic
+    /// (e.g. drop the early return) → this test panics instead of getting an Err.
+    #[test]
+    fn resolve_keys_before_scan_is_clean_device_not_ready() {
+        let mut session = DiscSession::from_parts_for_test(None, None, None);
+        let err = session
+            .resolve_keys(factory_of(|| HasUnitKey([1; 16])))
+            .expect_err("resolve_keys before scan must error, not panic");
+        assert!(
+            matches!(err, Error::DeviceNotReady { .. }),
+            "expected DeviceNotReady, got {err:?}"
+        );
+    }
 }