fix: a signal during party host rolls back and stops

The rollback trap was armed on EXIT INT TERM HUP with a body that did
not exit. After a signal the shell ran the cleanup, then resumed
cmd_host and kept configuring the party dir it had just removed.
Keep the cleanup on EXIT and make the signal traps exit with the
conventional 128+n status.

tests/95-stub-roundtrip.bats

@@ -39,6 +39,9 @@ case "$*" in
   *"new-session -d "*)
     # cmd_host expects the socket file to exist for apply_party_perms_file.
     [ -n "$sock" ] && : > "$sock"
+    # Optional injection point for the signal test: TERM the calling
+    # party process while it waits on us, mid-setup.
+    [ -n "${SIGNAL_ON_NEWSESSION:-}" ] && kill -TERM "$PPID"
     ;;
   *"display-message -p "*)
     # Caller is fishing for #{pid}; emit the sleeper's PID.
@@ -557,6 +560,21 @@ STUB
   [ ! -e "$party_dir" ]
 }
 
+@test "host interrupted by a signal rolls back and stops" {
+  # A signal trap body that does not exit lets the shell resume where
+  # it left off: cmd_host would clean up, then carry on configuring a
+  # party dir it just removed. The signal must end the run.
+  party_dir="$PARTY_SOCKET_DIR/party-$USER:sigterm.d"
+
+  SIGNAL_ON_NEWSESSION=1 run "$PARTY_BIN" host sigterm
+  [ "$status" -eq 143 ]
+
+  [ ! -e "$party_dir" ]
+  # Nothing but the rollback's kill-server follows new-session.
+  [ "$(sed -n '/new-session -d /,$p' "$TMUX_LOG" | sed 1d)" \
+      = "-S $party_dir/sock kill-server" ]
+}
+
 @test "host refuses leftover per-party dir without a roster" {
   # Simulate a prior incarnation that left a per-party dir behind with
   # no roster (the rollback trap couldn't fire — SIGKILL or panic).