Rewrite/backend v2

[httpsworldview]

Summary

• Redesign backend entirely. Virtual sink is gone, application capture should function identically, but the stream should never be interrupted upon graceful or forceful shutdown.
• Improves multichannel support and channel-aware DSP.
• Decouples DSP cadence from rendering and adds configurable frame rates.
• Reduces rendering allocations through instancing, caching, and buffer reuse (doesn't necessarily improve CPU time due to the above cadence changes, but other optimizations were performed)
• Fixes known precision and stale-cache bugs.

Related issues

Closes #20

Checklist

• I read CONTRIBUTING.md.
• Commits include a Signed-off-by: footer.
• I added or updated tests, or explained why not.
• I updated documentation when user-visible behavior changed.
• If AI tools were used, I reviewed the result carefully.