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/error.rs

@@ -60,6 +60,7 @@ pub const E_NO_STREAMS: u16 = 6009;
 pub const E_HALTED: u16 = 6010;
 pub const E_MAPFILE_INVALID: u16 = 6011;
 pub const E_UDF_BUFFER_TOO_SMALL: u16 = 6012;
+pub const E_UDF_NOT_FILESYSTEM: u16 = 6013;
 
 // AACS (7xxx)
 pub const E_AACS_NO_KEYS: u16 = 7000;
@@ -272,6 +273,14 @@ pub enum Error {
     UdfNotFound {
         path: String,
     },
+    /// The reader was addressable but the bytes are structurally NOT a UDF
+    /// filesystem — a deterministic tag/format mismatch (e.g. no Anchor Volume
+    /// Descriptor Pointer at sector 256, no partition descriptor, no File Set
+    /// Descriptor). Distinct from [`Error::DiscRead`] (a transient I/O fault):
+    /// this is a stable property of the media, not something a retry fixes. Lets
+    /// callers (notably FMTS key resolution) treat "not a UDF/FMTS disc" as a
+    /// clean negative while still failing loud on a real read fault.
+    UdfNotFilesystem,
     /// A `SectorSource` caller passed a destination buffer smaller than one
     /// 2048-byte sector. A contract violation on the public reader API —
     /// returned instead of panicking on the slice.
@@ -564,6 +573,7 @@ impl Error {
             Error::MplsParse => E_MPLS_PARSE,
             Error::ClpiParse => E_CLPI_PARSE,
             Error::UdfNotFound { .. } => E_UDF_NOT_FOUND,
+            Error::UdfNotFilesystem => E_UDF_NOT_FILESYSTEM,
             Error::UdfBufferTooSmall => E_UDF_BUFFER_TOO_SMALL,
             Error::DiscTitleRange { .. } => E_DISC_TITLE_RANGE,
             Error::IfoParse => E_IFO_PARSE,
@@ -1275,6 +1285,7 @@ mod tests {
             E_HALTED,
             E_MAPFILE_INVALID,
             E_UDF_BUFFER_TOO_SMALL,
+            E_UDF_NOT_FILESYSTEM,
             E_AACS_NO_KEYS,
             E_AACS_CERT_SHORT,
             E_AACS_AGID_ALLOC,