Fix three defects in last round's own fixes

Round 3 audited the round-1/2 fix commits rather than trusting them, and found
three defects in that new code. This is why the pin moves each round.

1. LICENCE REGRESSION, and it was mine. Reverting the "distinguish a failed key
   source" commit also restored a verbatim reference-decoder table citation in
   src/mux/codec/dts.rs, because both changes were in that one commit. The MIT
   licence cleanup was silently undone at HEAD and nothing caught it.

   The citation is replaced with ETSI TS 102 114 §5.3.1 again, and — more
   importantly — the rule now lives in the leak gate instead of in my memory.
   scan-secrets.sh gains LICENCE_RE, which flags ff_dca*, dcadec, l-smash,
   libav*, and bare ffmpeg/FFmpeg as REF-IMPL-CITATION. `no ffmpeg` is
   explicitly allowed via negative lookbehind: stating what this project does
   NOT depend on carries no risk and is a genuine selling point. Verified by
   re-introducing the citation (gate fails) and removing it (gate clean).

2. TsMuxer armed params_written even when the avcC/hvcC parser returned None, so
   a track whose codec_private exists but will not parse was muxed to BD-TS with
   no VPS/SPS/PPS ever emitted — undecodable video, reported as success, with no
   log line. Last round's fix corrected WHICH parser is used and left this half
   untouched. Arming the flag is still right (retrying identical bytes cannot
   succeed) but it is no longer silent: it now warns with the track, codec and
   codec_private length.

3. test_aes_cbc_roundtrip defined a LOCAL fn aes_cbc_encrypt that SHADOWED the
   production primitive, so it round-tripped a copy of the algorithm against
   itself and never touched crypto::aes_cbc_encrypt — the function this cycle
   added. Any mutation to the shipped code passed it. The shadow is deleted and
   the test now calls the real primitive; verified by mutating
   crypto::aes_cbc_encrypt, which now fails it and previously would not have.

src/mux/codec/dts.rs

@@ -691,9 +691,9 @@ const DTS_PCMBLOCK_SAMPLES: u32 = 32;
 const DTS_SUBBAND_SAMPLES: u32 = 8;
 /// Number of LEGAL `AMODE` (channel-arrangement) codes. The 6-bit AMODE field
 /// (ETSI TS 102 114 §5.3.1) has 16 defined channel arrangements, codes 0-15;
-/// only 16-63 are reserved/user-defined and undecodable. ffmpeg's
-/// `ff_dca_channels[16] = {1,2,2,2,2,3,3,4,4,5,6,6,6,7,8,8}` confirms all 16 are
-/// decodable — codes 10-15 are the 6/7/8-channel layouts. A frame is dropped
+/// only 16-63 are reserved/user-defined and undecodable. The per-AMODE channel
+/// counts in ETSI TS 102 114 §5.3.1 cover all 16, confirming they are decodable —
+/// codes 10-15 are the 6/7/8-channel layouts. A frame is dropped
 /// only when `audio_mode >= DTS_AMODE_COUNT` (i.e. a truly reserved 16-63 code);
 /// dropping a legal 10-15 multichannel core would silence recoverable audio.
 const DTS_AMODE_COUNT: u32 = 16;

src/mux/tsmux.rs

@@ -195,8 +195,22 @@ impl<W: Write> TsMuxer<W> {
                         Codec::H264 => avcc_to_annex_b(cp),
                         _ => hvcc_to_annex_b(cp),
                     };
-                    if let Some(params) = params {
-                        annex_b.extend_from_slice(&params);
+                    match params {
+                        Some(params) => annex_b.extend_from_slice(&params),
+                        // A codec_private that EXISTS but will not parse means no
+                        // VPS/SPS/PPS reaches the stream and it is undecodable.
+                        // Arming the flag below is still right — retrying the same
+                        // bytes on the next keyframe cannot succeed — but it must
+                        // not be silent, which is what it was: the mux reported
+                        // success while emitting parameter-set-free video.
+                        None => tracing::warn!(
+                            track,
+                            codec = ?self.video_codec[track],
+                            codec_private_len = cp.len(),
+                            "bd-ts: codec_private did not parse as an avcC/hvcC \
+                             record; no parameter sets emitted and the video will \
+                             not decode"
+                        ),
                     }
                 }
                 self.params_written[track] = true;