Fix audit findings: DTS AMODE bound, key-fetch negative memoization, PGS probe coverage

- dts: accept all 16 legal AMODE channel-arrangement codes (0-15), not just
  0-9. Per ETSI TS 102 114 the 6-bit AMODE field has 16 defined arrangements;
  only 16-63 are reserved. a reference decoder the spec per-AMODE channel table confirms 10-15 are
  decodable 6/7/8-channel layouts. The old bound of 10 dropped spec-legal
  multichannel core frames as undecodable, silencing recoverable audio. Add a
  regression test (literal 0..16 range) that fails if the bound reverts to 10.

- keysource: only memoize a NEGATIVE (empty) key-fetch result when every source
  genuinely ran and none held the key — never when a source Err'd (network down,
  unreachable). A transient outage was being cached as a permanent "no key" for
  the fingerprint, permanently dropping a unit that could be recovered once the
  source came back. Thread an `errored` flag out of the drivers and gate the
  cache insert on it. Tests cover both the recover-after-outage case and that a
  genuine absence is still memoized.

- pgs_forced_probe: add happy-path coverage feeding real synthetic BD-TS PGS
  display sets through the full demux -> parse -> observe -> apply path, both a
  forced verdict landing and a non-forced verdict clearing a vendor flag.

- mp4: correct fit_report doc (audio carried is AC-3/E-AC-3 AND DTS/DTS-HD).

- scan_iso test: add independent fixture expectations (volume id) so the parity
  test is no longer purely tautological against a re-run of the same composition.

src/mux/mp4/mod.rs

@@ -158,8 +158,9 @@ pub struct Mp4FitReport {
 }
 
 /// Compute the fit plan without opening a file. Video: the first primary
-/// HEVC/H.264 track. Audio: every AC-3 / E-AC-3 track. Everything else is
-/// skipped with a reason.
+/// HEVC/H.264 track. Audio: every track `audio::audio_fits` carries — the Dolby
+/// family (AC-3 / E-AC-3) and DTS (core / DTS-HD HRA / DTS-HD MA). Everything
+/// else is skipped with a reason.
 pub fn fit_report(title: &DiscTitle) -> Mp4FitReport {
     let mut included = Vec::new();
     let mut skipped = Vec::new();