fix(mux): parse the AAC program config element instead of guessing stereo

quest/m1/audio-codecs/README.md

@@ -37,14 +37,12 @@ mono/stereo.
 
 The core configuration and layout contracts land in main. These quests implement
 surround and backend dispatch on that contract; each platform then lands as
-its own decode and encode quest so verification stays per host. The
-HE-AAC refusal and the PCE parse are defects in what ships today and are
-ready now.
+its own decode and encode quest so verification stays per host.
 
 ## Quests
 
 - [HE-AAC refusal](/quest/m1/audio-codecs/he-aac-refusal.md) - implicit-SBR HE-AAC over TS is refused instead of half-decoded as the LC core
-- [AAC PCE](/quest/m1/audio-codecs/aac-pce.md) - a channel_config of 0 parses the program config element instead of guessing stereo
+- [TS export PCE](/quest/m1/audio-codecs/ts-export-pce.md) - a TS export of a PCE-described AAC track writes channel_config 0 and the PCE instead of a count-derived config
 - [Layout](/quest/m1/audio-codecs/layout.md) - the settled `Layout` carries up to 7.1 through decode, resample, playback, and the FFI
 - [Decode seam](/quest/m1/audio-codecs/decode-backend.md) - `decode::backend` selects a platform decoder before symphonia, mirroring moq-video
 - [AudioToolbox decode](/quest/m1/audio-codecs/decode-audiotoolbox.md) - macOS and iOS decode HE-AAC, multichannel AAC, and what else the framework offers

quest/m1/audio-codecs/he-aac-refusal.md

@@ -31,4 +31,3 @@ does too.
 ## Related
 
 - [Decode seam](/quest/m1/audio-codecs/decode-backend.md) - the sniff becomes the software backend's contract
-- [AAC PCE](/quest/m1/audio-codecs/aac-pce.md) - the other place the catalog lies about an AAC stream

quest/m1/audio-codecs/layout.md

@@ -16,7 +16,10 @@ Keep the representation extensible. Each codec module maps its native
 order into it: AAC's `C L R Ls Rs LFE` and Opus's Vorbis `L C R Ls Rs LFE`
 both become `L R C LFE Ls Rs`. An unspecified discrete PCM layout remains
 valid for passthrough but is refused for spatial remixing; never invent speaker
-positions from an arbitrary channel count.
+positions from an arbitrary channel count. An AAC stream with
+channelConfiguration 0 takes its layout from the program config element's
+front, side, back, and LFE elements, which `moq_mux::codec::aac` already parses
+for the channel count.
 
 - Reuse the settled PCM descriptors and codec/consumer settings. This quest
   adds supported layout behavior rather than replacing public field types.

quest/m1/audio-codecs/ts-export-pce.md

@@ -0,0 +1,17 @@
+# [S] TS export carries an AAC track's program config element
+
+## Goal
+
+`moq-mux`'s MPEG-TS export writes an AAC track described by a program config
+element (channelConfiguration 0) as ADTS with channel_config 0 and the PCE at
+the start of the first raw data block, so a TS round trip keeps the layout.
+Today the export derives the ADTS channel_config from the channel count, which
+mislabels such a track.
+
+## Plan
+
+Take the PCE from the track's AudioSpecificConfig description with the parser
+that reads it on import, and write it as ffmpeg does: once, leading the first
+raw data block. Tracks with a nonzero channelConfiguration are unchanged. Test
+a round trip of the quad fixture from the PCE import (`aac_quad.ts`): import,
+export, and import again, asserting the same description and channel count.