Fix the DVD MPEG-audio codec mapping and the fabricated AACS docs

parse_audio_attr mapped DVD audio_coding_mode 2 to Codec::Mpeg1 — the MPEG-1
VIDEO variant. Codec::kind() reports Video for it, so a DVD MPEG-audio stream
was classified and handled as video everywhere downstream. Modes 2 and 3 are
both MPEG audio Layer II (3 adds the MPEG-2 multichannel extension), so both
map to Codec::Mp2. A test now walks every coding mode and asserts each result's
kind() is Audio, so no mode can map to a non-audio codec again.

docs/aacs.md documented an entire keydb-resolving API that does not exist:
ScanOptions::with_keydb, Disc::open_title, reader.read_unit(). None of those
symbols appear anywhere in the crate, and ScanOptions has no keydb field — its
own doc comment says "libfreemkv is lookup-free — it resolves no keys". A
reader following that page would conclude the library reads keydb.cfg, which
inverts the actual design: the caller resolves keys out-of-band through a
KeySource and applies them with Disc::decrypt_with.

The section is rewritten against the real API, and the AacsState table's
`key_source` type corrected from KeySource to KeyOrigin.

Worth recording: the first replacement example I wrote was itself wrong. It
used `input("disc://...")`, which resolve.rs explicitly rejects with
Error::DiscUrlNotDirect — live disc must go through Drive::open + Disc::scan +
DiscStream::new. Every symbol and signature in the committed example was
checked against the source rather than assumed.

src/ifo.rs

@@ -571,10 +571,14 @@ fn parse_audio_attr(data: &[u8], offset: usize) -> Result<DvdAudioAttr> {
     let b1 = byte_at(data, offset + 1)?;
 
     let coding_mode = (b0 >> 5) & 0x07;
+    // DVD-Video audio_coding_mode. Modes 2 and 3 are both MPEG audio Layer II
+    // (3 adds the MPEG-2 multichannel extension), so both are Codec::Mp2.
+    // Mode 2 previously mapped to Codec::Mpeg1 — a VIDEO variant, so
+    // Codec::kind() reported Video and the audio stream was classified and
+    // handled as video everywhere downstream.
     let codec = match coding_mode {
         0 => Codec::Ac3,
-        2 => Codec::Mpeg1,
-        3 => Codec::Mp2,
+        2 | 3 => Codec::Mp2,
         4 => Codec::Lpcm,
         6 => Codec::Dts,
         _ => Codec::Unknown(coding_mode),
@@ -1218,6 +1222,33 @@ mod tests {
         assert_eq!(crate::mux::ps::dvd_audio_pid(0x89), Some(0xBD89));
     }
 
+    #[test]
+    fn every_audio_coding_mode_maps_to_an_audio_codec() {
+        // An audio attribute block must never yield a codec whose kind() is Video.
+        // Mode 2 (MPEG-1 audio Layer II) mapped to Codec::Mpeg1 — the MPEG-1 VIDEO
+        // variant — so a DVD MPEG-audio stream was classified as video downstream.
+        for (mode, want) in [
+            (0u8, Codec::Ac3),
+            (2, Codec::Mp2),
+            (3, Codec::Mp2),
+            (4, Codec::Lpcm),
+            (6, Codec::Dts),
+        ] {
+            let mut data = vec![0u8; 16];
+            data[0] = mode << 5;
+            data[2] = b'e';
+            data[3] = b'n';
+            let attr = parse_audio_attr(&data, 0).unwrap();
+            assert_eq!(attr.codec, want, "coding_mode {mode} must map to {want:?}");
+            assert_eq!(
+                attr.codec.kind(),
+                crate::disc::CodecKind::Audio,
+                "coding_mode {mode} produced {:?}, whose kind is not Audio",
+                attr.codec
+            );
+        }
+    }
+
     #[test]
     fn audio_attr_dts() {
         let mut data = vec![0u8; 16];