Finalize the `visualizer@v1` role

[marcelveldt]

There is already a visualizer@_draft_r1_support implemented in aiosendspin and Music Assistant, but that still has some not yet addressed quirks:

• Missing support for stream/request-format
• Does not differentiate between beats and downbeats
• Beats and other visualization data arrive on separate binary channels with no monotonicity guarantee, so clients have to merge by timestamp instead of trusting arrival order
• rate_max only applies to spectrum
• Multiple frames of data can get batched into a single message, while all other roles have a strict one frame per message approach. This makes the specification slightly more inconsistent
• Related to the previous point, servers are not required to do any batching. But requiring it would also make low latency applications more complicated

We also need to track down the WLED folks to get Sendspin implemented.