Remove the IPC socket of each shell a test starts (#13443)

* Answer the Elsewhen panel over the shell's IPC socket

The Elsewhen panel arrived with a plain IpcHandler, so omarchy-shell
reached it only through the slower qs ipc fallback, and the test that
every first-party handler registers as ShellIpc failed on quattro.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Remove the IPC socket of each shell a test starts

The runtime smoke and screenshot sanity tests start the real shell from a
temporary copy and kill it, which leaves its IPC socket behind in
XDG_RUNTIME_DIR. A session that runs the suite a few times collected
dozens of them.

base-test.sh gains shell_ipc_socket, which derives a shell's socket as
omarchy-shell does, and both tests remove theirs in cleanup. The socket
test derives its fixture's socket through it too, so the helper cannot
drift from omarchy-shell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/shell-ipc-socket-test.sh

@@ -22,8 +22,7 @@ touch "$root_dir/shell/shell.qml"
 # The shell names its socket after its config and display, as omarchy-shell
 # derives it.
 display="test-display"
-socket_id=$(printf '%s\n%s' "$root_dir/shell" "$display" | md5sum)
-socket="$run_dir/omarchy-shell-${socket_id:0:16}.sock"
+socket=$(XDG_RUNTIME_DIR="$run_dir" WAYLAND_DISPLAY="$display" shell_ipc_socket "$root_dir")
 
 # qs is the fallback: it records that it ran and answers.
 cat >"$stub_bin/qs" <<'SH'