Resolve server-side spec gaps

[maximmaxim345]

The PR with server-implementation-breaking changes I mentioned in #113. Clients do not have to adjust anything for these; they tighten what a conforming server must do.

Specifically:

• Role activation: a server MUST NOT activate a role or version the client did not list in supported_roles.
• Forward compatibility: servers (not just clients) MUST ignore unrecognized payload fields, and MUST NOT send fields the spec does not define.
• stream/request-format: a request the server cannot honor is not an error; the stream continues in a configuration the client supports. For players, stream/start MUST use a format from the client's supported_formats, so the server can never start a stream the client cannot decode.
• Reconnect backoff: after repeated consecutive Noise-handshake failures, the server SHOULD NOT keep auto-reconnecting until there is reason to expect success (operator re-initiates pairing, network conditions change).
• Artwork late join: after an artwork stream/start (initial or post-reconnect), the server SHOULD immediately send the current image for each channel whose source is not 'none', so a client joining mid-track is not left blank until the next track change.