Progressive message transmission and display

[or-else]

Add an option to transmit messages progressively, in chunks, in a way useful for LLM-generated messages.

It's currently possible to send messages progressively but it's a relatively heavy weight process: each message version has to contain complete message to show, each version is stored in DB.

Instead of the above, send message deltas/diffs, do not store incomplete messages in DB.

[or-else]

Current plan:

1. When the progressive message is created, a placeholder message with initial content is sent as a regular {pub} with a newly introduced header chunk: 1. The server responds the usual {ctrl} with the message's seq ID.
2. As LLM (or user) updates the message, client sends {note} packers with content diffs, chunk counter and seq returned in step 1. All attached sessions receive corresponding {info} packet.
3. An in-memory copy of the message is maintained at the server. It's updated every time a {note} is received. Whenever a subscriber requests messages with {get what=data}, the in-memory version is returned with the current chunk: X counter.
4. Once the LLM or user finishes editing the message, the message with replace header is sent with the full content of the message (the same as normal message editing).