### What changed?
Adds optimistic rendering for newly locally posted messages in channels
and threads.
- Messages appear immediately
- Relay echoes and history are deduplicated by event ID
- Rejected or timed-out publishes erase the optimistic rendering.
The implementation covers reconnect and hydration races, channel-window
and legacy WebSocket paths, thread-local overlays, and rapid concurrent
sends.
### Why?
This is a valuable partial mitigation for
[BOT-1449](https://linear.app/squareup/issue/BOT-1449/buzz-mobile-posted-messages-dont-appear-until-leavingre-entering-the):
senders no longer depend on receiving a relay echo before seeing their
own post.
It does not address the likely primary cause of stale channels. Mobile
currently does not recover live subscriptions after a rate-limited relay
`CLOSED`; that recovery is being handled separately.
### How is it tested?
Full mobile suite: 676 passed, 1 skipped.
Added regression coverage for optimistic insertion, authoritative
deduplication, rollback, reconnect and hydration, thread replies, rapid
and equal-time sends, never-echoed successful sends, and legacy
WebSocket retirement.
---------
Signed-off-by: npub102wg7q285p64ch2fjvstmf2ntn2sz3c4u5hmwatalc76mhsuauysftjtfj <7a9c8f0147a0755c5d499320bda5535cd5014715e52fb7757dfe3dadde1cef09@buzz.block.builderlab.xyz>
Co-authored-by: npub102wg7q285p64ch2fjvstmf2ntn2sz3c4u5hmwatalc76mhsuauysftjtfj <7a9c8f0147a0755c5d499320bda5535cd5014715e52fb7757dfe3dadde1cef09@buzz.block.builderlab.xyz>
