feat(decoder): Add real-time Stereo-to-7.1 Upmixing Engine to IAMF Renderer

[AytacDoyran]

Summary

This pull request introduces a lightweight, low-latency, and royalty-free Stereo-to-7.1 Upmixing Engine directly into the IamfDecoder architecture.

As legacy stereo assets still dominate streaming catalogs (e.g., YouTube), this implementation provides a standardized way to dynamically expand 2.0 channel PCM input into an immersive 8-channel (7.1) spatial layout without altering downstream bandwidth or requiring multi-channel server side re-encoding.

Technical Implementation

The upmix algorithm operates in the time-domain using explicit channel separation principles to avoid artificial phasing effects:

• Center (C) Extraction: Identifies in-phase signals between Left and Right channels via mid-side matrix analysis.
• Front (L/R) Management: Balances original frequency responses while shifting ambient side-components.
• Surround Side (Ls/Rs) & Back (Lb/Rb): Captures out-of-phase ambient content and applies discrete 20ms and 40ms circular delay lines to produce natural spatial depth.
• LFE Channel: Handles low-frequency routing for subwoofers below the crossover point.

Specifications & Optimization

• Google C++ Style Guide Compliant: The class definitions, variable namings (snake_case_), and namespace structural guidelines are strictly followed.
• Zero Allocation: Memory allocations for delay circular buffers happen exclusively on class initialization, ensuring zero overhead during the real-time processing loop.
• Licensing: Fully compatible with AOMedia's Royalty-Free Patent License Agreement. No third-party dependencies are introduced.

[jwcullen]

Thank you for your interest in IAMF.

We are going to close this PR because the IamfDecoder class is designed to strictly be an implementation of this specification, we want to decouple any supplementary processing. Additionally, this a very large feature that would take significant time to review and maintain. Architecturally, it seems that upmixing would be better handled as a stage of processing after the native IamfDecoder output, rather than be bundled with it.