Fold stream/request-format into client/state

[maximmaxim345]

stream/request-format was a request in name only: a decline is invisible (the stream continues and the request MUST NOT be treated as an error), so a client only ever learns the operative configuration from stream/start, and with no stream active the server already had to remember the requested format as de facto state. Per the CONTRIBUTING rule that fields expected to change during a connection belong in client/state, the message is dropped and its fields move there.

Changes

player gains an optional format preference in its state object; absent means the server selects per the supported_formats priority order, a present preference must be one of the listed entries (which also removes the previously undefined partial-request case), and the server may still deviate, for example to match a track's native sample rate or to apply an operator-configured format. artwork moves the per-channel source/format/width/height into a state channels array and artwork@v1_support is removed entirely: the array is positional from channel 0, an uncovered channel is source: 'none' so clients may truncate after their last active channel, an array longer than 4 is a protocol error, and format/width/height are required only when source is not 'none'. visualizer moves types, rate_max and spectrum into state; its hello object keeps only buffer_capacity. stream/start now derives each role's configuration from the hello constants plus the role's current state, answers a state change on an active stream with a new stream/start when the configuration changed, never starts a stream in response, and should wait for the state update a role (re-)activation requires before starting that role's stream.

Breaking changes

The stream/request-format message is removed; clients that send it break, and servers drop the handler. Artwork clients no longer send artwork@v1_support and instead carry their channel configuration in client/state; visualizer clients keep only buffer_capacity in their support object and carry the rest in client/state. Players are affected only if they used stream/request-format; the new format state field is optional.

Conflicts with #177 in the player state object; whichever lands second needs a rebase.

Closes #190