Coalesce console output

[scramsby]

The ET client wrote each received TERMINAL_BUFFER packet to stdout individually. When the server sent a burst of output (e.g. a tmux session switch redrawing multiple panes), each packet became a separate write() to the local terminal, causing the terminal emulator to render intermediate states (screen-clear in one frame, repaint in the next) — producing visible flicker.
Fix: accumulate all TERMINAL_BUFFER payloads received in a single select() wake into one string, then write once after draining the packet queue.

To test:
Connect to Linux machine from Windows machine using ET.
Switch between tmux sessions with sidebar panes — verify no flicker and no partial screen erasure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.90%. Comparing base (ddf67a5) to head (483028f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
- Coverage   86.95%   86.90%   -0.06%     
==========================================
  Files          70       70              
  Lines        5282     5284       +2     
  Branches      490      491       +1     
==========================================
- Hits         4593     4592       -1     
- Misses        689      692       +3     

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