fix(es-level): remove altGroup from language-variant audio tracks

altGroup carries mutually exclusive switching semantics in §9 (PCR
discontinuity, STC re-init, PAT/PMT re-parse on switch). Simultaneous
subscription to multiple tracks in the same altGroup contradicts that
definition. Language variants are also independent services, not ABR
alternate renditions, so altGroup is the wrong mechanism.

Subscribers select the audio language of their choice among the
published ES-level tracks using m2tsProgramNumber and role; no grouping
field is required.

* §5.5.3: remove altGroup paragraph
* catalog example: remove altGroup from both audio tracks; update
  description to drop "alternate" framing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gwendalsimon

draft-gregoire-moq-msfts.md

@@ -366,10 +366,6 @@ PCR from the track where `m2tsPcrPid` equals `m2tsEsPid`.  When `m2tsMuxRate`
 is declared on the PCR-bearing track, the subscriber SHOULD use that value to
 restore the original mux rate in the combined output.
 
-Audio ES-level tracks for the same program MAY share an `altGroup` to indicate
-that they are alternate language renditions.  A subscriber MAY subscribe to
-more than one audio track within an alternate group and include all subscribed
-audio PIDs in the reconstructed PMT.
 
 ## PCR and Timing {#pcr-timing}
 
@@ -841,11 +837,11 @@ PAT and PMT on the new track before routing packets to a decoder.
 ## ES-Level Tracks - Per-Elementary-Stream Publishing {#example-es-level}
 
 This example shows a live program published as separate ES-level tracks: one
-video track carrying the PCR, two alternate audio tracks (English and Spanish),
-and one Event Information Table track.  The video and audio tracks MUST have
-synchronized Group boundaries.  A subscriber combining video and one audio
-track constructs a PAT and PMT listing the two subscribed PIDs and sources PCR
-from the video track (PID 257).
+video track carrying the PCR, two audio tracks for different languages (English
+and Spanish), and one Event Information Table track.  The video and audio
+tracks MUST have synchronized Group boundaries.  A subscriber combines the
+video track and the audio track of its choice by constructing a PAT and PMT
+listing the subscribed PIDs and sourcing PCR from the video track (PID 257).
 
 ~~~ json
 {
@@ -879,7 +875,6 @@ from the video track (PID 257).
       "role": "audio",
       "mimeType": "video/mp2t",
       "bitrate": 128000,
-      "altGroup": 2,
       "m2tsPacketSize": 188,
       "m2tsPacketsPerObject": 32,
       "m2tsProgramNumber": 1,
@@ -897,7 +892,6 @@ from the video track (PID 257).
       "role": "audio",
       "mimeType": "video/mp2t",
       "bitrate": 128000,
-      "altGroup": 2,
       "m2tsPacketSize": 188,
       "m2tsPacketsPerObject": 32,
       "m2tsProgramNumber": 1,