Chathistory seeding and scroll-back paging

[zealsprince]

draft/chathistory is requested and tracked, but the runtime never issues a CHATHISTORY command: a channel renders only what arrives live after join. Opening a channel should seed from history, and scrolling up should page further back.

Spec

• Uplink - connection flow
• Local Cache - the paging knobs this later sits behind

Acceptance criteria

• Opening a channel issues CHATHISTORY LATEST and renders the batch
• Scrolling to the top issues CHATHISTORY BEFORE the oldest rendered msgid and prepends
  • This is already intermingled with the actual UI; we should seek to test this in isolation so as to not have a dependency on a full UI implementation.
  • There are more considerations to this implementation case as the UI matures and evolves (images, media, etc)
  • We should have an example case of a list of messages in a scrollable container, where scrolling up issues CHATHISTORY BEFORE and pull the next batch of messages.
• Replayed batches produce no notification or unread side effects
• Without the cap, channels degrade to live-only with no error surfaced

Out of scope

• Cache-first paging and prefill (Clients: local history cache #10)
• Reconnect catch-up (its own ticket)
• Calculating media heights and various other nuanced aspects of scrollback behavior ([FLAG] MISSING TICKET)

Blocks reconnection catch-up and the local history cache epic (#10). Blocked by nothing.

([FLAG] MISSING UI TICKET)