Message buffer ordering and dedup

[zealsprince]

The parsing half exists: incoming messages already extract msgid and server-time into the message struct. Two gaps. First, both fields are unwrapped with .expect("unimplemented"), so any line without them (presence events, older servers) hits a panic path. Second, nothing uses them: the buffer appends in arrival order and nothing dedupes.

Key the buffer by msgid and sort by server-time. Live delivery, echo-message self-copies, and chathistory replays of the same message merge into one record; keyless lines collapse by the synthetic signature from the cache spec's dedup rules, which apply to the in-memory buffer the same way.

Spec

• Local Cache - dedup rules
• Tags

Acceptance criteria

• A message seen live and again in a replay renders once
• The sender's echo-message copy merges with the locally rendered send
• Lines without msgid or server-time neither panic nor duplicate; they collapse by the type + author + text + time signature
• An old-stamped line arriving live (event-playback) inserts at its server-time position, not the tail
• This goes hand-in-hand with Chathistory seeding and scroll-back paging #33 from a testing perspective since we should be able to POC this feature there without building a separate test-fixture

Out of scope

• Persistence (Clients: local history cache #10)

No blockers. Rich messaging (#7) assumes this is in place.