Add a regression test that exercises the code path which crashed in
v0.46.2 (#10017): when paused_rendering is active and a selection
extends into the scrollback, the inner loop of apply_selection iterates
with a negative y. Without the recently-added paused_y translation and
the paused_y < 0 guard, the call to linebuf_init_line treats the
negative y as a huge unsigned index_type and reads ~4GB out of bounds
in line_attrs[idx], crashing with SIGBUS.
The test reproduces the trigger deterministically via the Screen Python
bindings and asserts that current_selections() returns the expected
buffer instead of crashing.
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