feat(helper): make install-helper.sh systemd-optional (OpenRC/Artix, runit)

[hikaps]

Closes #55 (tracks #54 — the Artix user who couldn't install).

Change

The helper is Type=dbus with BusName=io.github.hikaps.CouchPlayHelper, so "running" just means owning that name on the system bus — D-Bus can activate it on demand without a systemd unit. install-helper.sh now detects systemd (has_systemd: /run/systemd/system + systemctl) and branches:

• systemd (unchanged): installs the unit, daemon-reload, enable, restart.
• non-systemd: skips the unit + all systemctl calls; strips SystemdService= from the installed D-Bus service file (source untouched) so dbus activates via Exec= directly; reloads dbus via the existing SIGHUP fallback in reload_dbus(); warns that polkitd must be running (rc-service polkitd start / sv start polkitd).

status() reports D-Bus activated on demand (no systemd) without calling systemctl on non-systemd; uninstall() guards daemon-reload; reload_dbus() gates its systemctl reload dbus on has_systemd (the SIGHUP fallback already covered non-systemd).

Tradeoff

Without the systemd unit the helper loses the sandboxing directives (ProtectSystem, etc.) — acceptable, since the helper runs as root and needs broad access (/etc, /home, /dev/input), and most are already relaxed (ProtectSystem=false, ProtectHome=false).

Acceptance (#55)

• install-helper.sh install succeeds on OpenRC/Artix with no systemctl present. ✓ (logic; non-systemd path skips all systemctl calls)
• Launching the GUI D-Bus-activates the helper. ✓ (SystemdService= stripped → dbus Exec= activation)
• status reports correctly without systemctl. ✓ (verified via simulation: D-Bus activated on demand, exit 0)

Verification

• bash -n clean; shellcheck zero findings.
• systemd-path regression on a Bazzite host: has_systemd=true, status → Running + D-Bus Available (unchanged behavior).
• non-systemd simulation (force has_systemd=false): status → D-Bus activated on demand (no systemd), no systemctl error.
• sed-strip confirmed: installed .service keeps [D-BUS Service]/Name/Exec/User, drops only SystemdService=.
• No C++ changes; unit suite unaffected.