craft-audio: review fixes, round 5 (#443)

No change in decoding. An aac-encode build in the next PR needs the
split_adts_upto re-export behind aac-decode, and ID_END only in tests.

Docs say the lookahead's headers give an ADTS stream's configuration, that AAC in
CAF still goes through symphonia, and that ProgramConfig is not #[non_exhaustive].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/audio/README.md

@@ -409,7 +409,8 @@ What else an app adjusts:
   `AudioBuffer::from_interleaved` and `num_channels`, and the `ChannelLayout` constructors and
   `channels()`.
 - `AudioInfo`, `AudioError`, `EncodeOptions`, `BwfInfo`, `SampleFormat`, `BitDepth`, the codec
-  `Error` enums and the codec modules' header, frame and config structs are `#[non_exhaustive]`.
+  `Error` enums and the codec modules' header, frame and config structs (but for
+  `aac::ProgramConfig`) are `#[non_exhaustive]`.
   `Speaker::ALL` is a slice.
 - `TrackInfo::sample_rate` and `channels` are `Option`s.
 - Features are per codec. SoundCraft's `encode` (WAV, AIFF, FLAC) is now `lossless-encode`, made
@@ -419,6 +420,6 @@ What else an app adjusts:
   short slices instead of filling them with silence.
 - `DecodeOptions` is not `Copy`.
 - The FLAC encoder's vendor string is `craft-audio <version>`.
-- `decode` tries native decoders first. AAC no longer goes through symphonia.
+- `decode` tries native decoders first. AAC no longer goes through symphonia, except in CAF.
   MP4 and Matroska files decode to presentation length. A packet that fails is concealed, not
   dropped. A failed source read is `AudioError::Io`.

crates/audio/src/aac/decoder.rs

@@ -17,6 +17,7 @@ const ID_LFE: u32 = 3;
 const ID_DSE: u32 = 4;
 const ID_PCE: u32 = 5;
 const ID_FIL: u32 = 6;
+#[cfg(test)]
 const ID_END: u32 = 7;
 
 /// `extension_type` of a fill element carrying SBR data (ISO/IEC 14496-3 §4.5.2.8.2, Table 4.121).

crates/audio/src/aac/mod.rs

@@ -26,6 +26,7 @@ mod mdct;
 mod tables;
 mod tns;
 
+#[cfg(feature = "aac-decode")]
 pub(crate) use config::split_adts_upto;
 pub use config::{AdtsHeader, AudioSpecificConfig, ElementType, ProgramConfig, config_layout, split_adts};
 pub use decoder::Decoder;

crates/audio/src/track.rs

@@ -17,7 +17,7 @@ enum Inner {
         dec: Box<aac::Decoder>,
         up: Option<aac::Upsample2x>,
         order: Option<&'static [usize]>,
-        /// The format ADTS packets must keep, when the first ADTS header gave the configuration.
+        /// The format ADTS packets must keep, when their headers gave the configuration.
         adts_format: Option<AdtsFormat>,
     },
     Pcm {
@@ -92,7 +92,7 @@ impl TrackDecoder {
             #[cfg(feature = "aac-decode")]
             CodecId::Aac | CodecId::HeAac => {
                 // ADTS-framed packets (MPEG-TS, raw `.aac`) carry their configuration in each
-                // header: without extradata, the first header gives it.
+                // header: without extradata, the headers of the lookahead give it.
                 let payloads: Vec<&[u8]> =
                     lookahead.iter().map(|p| adts_frames(p).and_then(|f| f.first().map(|(_, unit)| *unit)).unwrap_or(p)).collect();
                 let mut adts_format = None;
@@ -347,7 +347,7 @@ pub(crate) fn adts_format_of(h: &aac::AdtsHeader) -> AdtsFormat {
     (h.object_type, h.sf_index, h.channel_config)
 }
 
-/// The raw access units of an ADTS-framed packet. In a stream whose first header gave its `format`,
+/// The raw access units of an ADTS-framed packet. In a stream whose headers gave its `format`,
 /// a frame that changes it would play wrongly, so it is malformed; with extradata, that decides the
 /// format and headers are not compared with it. Frames with several raw data blocks are
 /// unsupported.