Alt+Up pops the most recently queued follow-up (LIFO) from
_followup_queue, appends it to the current input with a newline---
separator, and marks it cancelled so process_loop skips it.

Repeated Alt+Up recalls one at a time until queue is empty.
_cancelled_followups set is checked in process_loop and discarded
on match to avoid sending the recalled message twice.

Addresses review feedback from britrik (NousResearch#4788):

- Replace text-based cancellation with UUID tags — identical messages
  queued twice no longer cancel each other incorrectly
- Wrap Alt+Enter payloads as {_followup_tag, payload} dicts so
  process_loop can identify followup items by ID, not content
- Fix phantom _followup_queue pops: display sync now only happens
  for tagged (Alt+Enter) items, not regular Enter messages
- _cancelled_followups stores UUIDs (bounded, auto-discarded on match)

Note: the image-payload cancel check was already correct in the original
— both sides extracted text via payload[0] — but UUID tagging makes the
intent unambiguous regardless of payload shape.