feat: add multi-audio track metadata support to Multiplex provider

[nums]

feat: add multi-audio track metadata support to Multiplex provider

Summary

Extends the Multiplex provider's TrackMap to support per-track audio metadata (PublicName, Language, Characteristics) and an AudioIndex field to differentiate multiple audio tracks sharing the same SourceTrackName. This brings multi-language audio support to the Multiplex provider, on par with what the Scheduled provider already offers.

Implemented with the help of Claude (Anthropic AI assistant).

What this does

• Adds PublicName, Language, Characteristics and AudioIndex fields to NewTrackInfo in MultiplexProfile
• When multiple audio tracks in a source stream share the same variant name, AudioIndex (0-based) selects which occurrence receives which metadata
• Without AudioIndex, a single <Track> entry acts as a catch-all and preserves the original track metadata from the source stream
• Both XML (.mux files) and JSON (REST API) formats are supported
• Metadata is applied at track creation time and flows through to LL-HLS
  EXT-X-MEDIA attributes (NAME, LANGUAGE, CHARACTERISTICS)
• Updates documentation with a multi-audio example

Why it's needed

The Multiplex provider is commonly used to combine streams for ABR delivery. Sources such as SRT streams or broadcast feeds often carry multiple embedded audio tracks. Without this feature, only one audio track per source variant could be mapped (additional tracks sharing the same SourceTrackName were silently dropped) making it impossible to expose multiple language tracks simultaneously in the LL-HLS master playlist.

Files changed

• providers/multiplex/multiplex_profile.h - added fields to NewTrackInfo, changed _new_track_infos_map to support multiple entries per SourceTrackName (vector), updated GetNewTrackInfo signature to accept an occurrence index
• providers/multiplex/multiplex_profile.cpp - XML/JSON parsing and serialization for the new fields
• providers/multiplex/multiplex_stream.cpp - occurrence tracking per variant name, apply metadata overrides when cloning tracks
• docs/live-source/multiplex-channel.md - added multi-audio example with AudioIndex, PublicName and Language

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

Adds multi-audio track metadata mapping to the Multiplex provider so multiple audio tracks sharing the same source variant name can be exposed with distinct HLS EXT-X-MEDIA attributes (e.g., language/name/characteristics).

Changes:

• Extend MultiplexProfile::NewTrackInfo with PublicName, Language, Characteristics, and AudioIndex, and update TrackMap to store multiple entries per SourceTrackName.
• Update XML/JSON parsing + serialization to read/write the new per-track audio metadata fields.
• Update MultiplexStream cloning logic to apply metadata overrides per occurrence and improve log output; update docs with a multi-audio example.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/projects/providers/multiplex/multiplex_stream.cpp	Tracks per-variant occurrence and applies per-track audio metadata overrides during track cloning.
src/projects/providers/multiplex/multiplex_profile.h	Expands NewTrackInfo, changes TrackMap to vector per source name, and adds occurrence-aware lookup.
src/projects/providers/multiplex/multiplex_profile.cpp	Adds XML/JSON read/write support for new metadata + AudioIndex, and updates serialization loops for vectors.
docs/live-source/multiplex-channel.md	Documents configuration for multi-audio mapping using AudioIndex and audio metadata fields.

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[getroot]

@nums
Thank you for submitting the PR.

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