fix: flush and unmount even when the sync is interrupted

rsync itself is safe under interruption. It writes to a hidden temporary file
and renames it into place only once complete, and by default deletes any
partial file when interrupted -- verified both in the manual and by killing a
transfer and inspecting what was left, which was nothing. --partial is
deliberately absent and there is now a test asserting it stays that way. An
unclean kill can leave a hidden .track.mp3.XXXXXX behind; it is unplayable, it
is not in the source, and the next run's --delete removes it.

The script was not safe. Ctrl-C killed it before the sync and the unmount,
leaving a journal-less FAT filesystem holding dirty buffers -- which is the
exact corruption the script exists to prevent, arrived at by the most likely
route a person would take.

INT and TERM are now trapped. Both the normal path and the interrupt path call
the same finish function, so the flush and the unmount cannot drift apart, and
an interrupted run exits 130 rather than pretending to have succeeded.

The test is structural rather than timed. Reproducing a mid-transfer signal
needs a payload large enough to be slow, and a test that depends on winning a
race is a test that fails in CI for reasons that have nothing to do with the
code. The behaviour was verified by hand: SIGTERM mid-transfer gave exit 130,
the flush ran, and the destination held no short files and no leftover
temporaries.

tests/test_sync_to_ipod.py

@@ -204,3 +204,27 @@ def test_directory_timestamps_are_not_set(mirror, tmp_path):
     script = SCRIPT.read_text()
 
     assert "--omit-dir-times" in script
+
+
+def test_an_interrupt_is_trapped_so_the_filesystem_is_flushed():
+    """Ctrl-C during a transfer would otherwise skip the sync and the unmount,
+    leaving a journal-less FAT filesystem with dirty buffers -- which is the
+    corruption this script exists to prevent.
+
+    Structural rather than timed: reproducing a mid-transfer signal needs a
+    payload large enough to be slow, and a test that depends on losing a race
+    is a test that fails in CI for no reason.
+    """
+    script = SCRIPT.read_text()
+
+    assert "trap interrupted INT TERM" in script
+    assert "exit 130" in script
+
+
+def test_the_flush_and_unmount_happen_on_every_exit_path():
+    script = SCRIPT.read_text()
+
+    # Both the normal path and the interrupt path go through the same function,
+    # so one cannot drift from the other.
+    assert script.count("finish\n") >= 2
+    assert "--partial" not in script, "rsync must delete partial files, not keep them"