Migrate mixing/decoding toolchain to Symphonia

[FelixMcFelix]

This issue proposes that we finally move away from (purely) IPC-based decoding to the codec and framing stack in development in Symphonia.

This would be an involved piece of work, involving (at least):

• Write an audiopus-based Symphonia codec, and port the DCA framing mechanism. The audiopus-based codec cannot be upstreamed, as Symphonia requires pure-Rust implementations. Similarly, this is not on their current roadmap.
• Implement DASH handling for youtube and other multimedia handlers extracted from youtube-dl.
• Investigate upstreaming MKV support to Symphonia. The MKV muxer underlies formats such as WebM (used primarily by youtube et al.).
• Ensure that Opus packet passthrough can be performed through Symphonia.
• Port our stream-cacheing wrappers to this ecosystem.

There are several gaps here, but it's also an exciting project that should help us leverage their existing (and forthcoming) codec implementations.

[FelixMcFelix]

Closed by #89.