fix: pre-existing renderer regression tests terminal-size independent

The two scroll-anchor regression tests used fresh_with_lines(50) which
didn't work on terminals taller than ~56 rows (visible_lines > 50). On
such terminals max_offset = total - visible = 0, so scroll_line_up is
a no-op — the test's scroll_offset never changed and assertions
failed with 'left: 1, right: 0' or 'left: 3, right: 133'.

Fix: fresh_with_lines_scrollable(n, margin) pads the buffer to at
least visible_lines + margin lines so scrolling always has room
regardless of terminal height. The replace_from test now uses buffer-
relative indices instead of hardcoded '40' since the buffer size varies.
Also cleaned up debug eprintln left in from earlier investigation.

Author: Yogthos