tests: cover the roster and the command line

The roster tests pin down the .old backup rotation and the five minute
periodic save, with a fake clock in place of datetime.

The command line tests call main() in-process with the save and data
paths in a temporary directory, so coverage counts them. They cover
listing, loading, importing and exporting saves, dumping and loading
word lists, and the first-run character creation. The curses UI is
swapped for a stub there so a stray code path fails instead of taking
over the terminal.

--load-save 0 falls through to the menu and --load-save -1 plays the
last character instead of being rejected; both are marked xfail.

A shared fixture restores pytest's SIGINT handler after the UIs
install their own.

tests/conftest.py

@@ -1,4 +1,16 @@
+import signal
 import sys
+from collections.abc import Iterator
 from pathlib import Path
 
+import pytest
+
 sys.path.insert(0, str(Path(__file__).parent))
+
+
+@pytest.fixture(autouse=True)
+def _restore_sigint() -> Iterator[None]:
+    """The UIs install their own SIGINT handler; keep pytest's."""
+    handler = signal.getsignal(signal.SIGINT)
+    yield
+    signal.signal(signal.SIGINT, handler)