Three defences against cursor-position desync in the two-line progress bar:

A) _post_with_retry now accepts an on_warn callback. The heartbeat flush
   closures pass _render_warn, which acquires _render_lock before printing,
   writes \n to leave the progress lines, and resets display["lines"]=0 so
   the next render starts fresh. Previously a brief server hiccup would print
   a raw \n outside the lock, shifting the cursor and orphaning both progress
   lines to scrollback.

B) SIGWINCH handler: resets display["lines"]=0 on terminal resize so cursor
   tracking recovers on the next heartbeat cycle.

C) Pre-scroll the initial single→two-line transition: prepend \n\x1b[1A so
   line2's row is guaranteed to exist below line1 even if the cursor is on
   the terminal's bottom row when two-line mode first activates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>