craft-audio: document the ADTS-in-MP4 seek and edit limits (#443)

AAC in MP4 stored as ADTS frames, with sample durations that count fewer
frames than the samples hold, decodes in full from the start, but seeks
place samples by those durations and land later than asked, and an edit
list measured in them can end the track early. The README's Limits section
now says so, and the seek example points to it.

Reported by FredApps.

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

crates/audio/README.md

@@ -131,7 +131,7 @@ use craft_audio::{DecodeOptions, Decoder};
 
 let file = std::fs::File::open("long-recording.m4a")?;
 let mut dec = Decoder::open(file, Some("m4a"), &DecodeOptions::default())?;
-dec.seek(48_000 * 60)?;                   // sample-exact: the next block starts here
+dec.seek(48_000 * 60)?;                   // sample-exact (see Limits): the next block starts here
 while let Some(block) = dec.next_block()? {
     // block.channels: planar f32, WAV channel order
 }
@@ -314,6 +314,11 @@ match craft_audio::decode(&bytes, None) {
 - MP4 edit lists: a single media edit (encoder priming and the end) is applied, alone or after one
   empty edit; the empty edit's delay is not added as silence. Other edit lists are not applied, and
   the media plays as it is stored.
+- AAC in MP4 stored as ADTS frames (not standard), when the sample durations count fewer frames
+  than the samples hold (HE-AAC's core frames, or several frames to a sample): the track decodes in
+  full when read from the start, but `Decoder::seek` places samples by those durations, so it lands
+  later than asked, and an edit list measured in them can end the track early. Telling would mean
+  reading every sample.
 
 ## Codecs