improvise-up8g: a headless run's extent is fixed, not read off the terminal

`run_headless_commands` read `crossterm::terminal::size()`, and the page size
decides where `page-scroll` lands, so one script wrote a different record in a
different window — and CI (no tty, 80x24 default) disagreed with a shell. Pin
it to 80x24 in `HEADLESS_EXTENT`. The test drives two sizes through LINES/
COLUMNS with the child detached by `setsid`, and asserts the same record.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

README.md

@@ -210,6 +210,10 @@ improvise cmd 'formula Product "Total = Widget + Gadget"' -f model.improv
   exist) is printed but still exits 0, because a status message carries no
   severity for the exit code to read. Check stderr as well as `$?` when it
   matters.
+- **A run behaves as if its terminal were 80x24, whatever it runs in.** A page
+  of rows is a real distance — `page-scroll` moves the cursor by one — so
+  reading the actual window would make the same script write a different record
+  in a different terminal, and a different one again in CI.
 - **A run that changed nothing does not rewrite the document.** Writing it back
   regardless silently normalises it — `1e20` becomes its twenty digits — which
   turns a hand-edited fixture into a no-op on the next run.