ofonod: GNOME Calls backend over org.ofono for Phosh and GNOME Mobile

imsd-ofonod presents imsd on the system bus as an oFono modem — Manager at
/, Modem + VoiceCallManager at /imsd, a VoiceCall object per call — which
is what GNOME Calls' bundled ofono provider drives; Calls, Phosh and GNOME
Shell run unmodified. org.gnome.Calls is not a seam a third party can
provide (Calls exports it, nothing feeds it), and Calls' provider plugins
link private headers, so org.ofono is the only D-Bus contract available.
Pure GDBus translation of net.catcrafts.IMS1, no core, like imsd-dialerd.

Packaged as the opt-in imsd-ofono subpackage: the daemon, its unit
(Conflicts=ofono.service), the system-bus policy and a gschema override
that points Calls at the ofono provider instead of mm (the ModemManager
origin would offer the modem's CS voice path, which carries no audio on
these phones). The policy grants root send_destination=org.ofono: under
dbus-broker a broadcast is checked against the sender's send policy for
every name the receiver owns, so without it imsd's call signals never
reach the daemon.

Verified on a Fairphone 6 with GNOME Calls 50.0: outgoing and incoming
calls with audio both ways, answered and hung up through
org.gnome.Calls.Call. Known Calls-side gap documented in the README: after
the VoiceCallManager interface is withdrawn and re-added, Calls keeps the
old origin's CallAdded handler and eventually crashes.

packaging/imsd-ofonod.service

@@ -0,0 +1,25 @@
+# SPDX-License-Identifier: GPL-3.0-only
+# SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+[Unit]
+Description=oFono-compatible GNOME Calls backend for imsd
+Documentation=https://forgejo.catcrafts.net/Catcrafts/imsd
+# org.ofono is a system-bus name, so this is a system service; it needs no
+# privileges beyond owning that name. Only enable it on a GNOME/Phosh session
+# — Plasma Mobile uses imsd-dialerd instead. Both may run at once (different
+# buses, different consumers), but a real ofonod must not.
+Conflicts=ofono.service
+After=imsd.service
+Wants=imsd.service
+
+[Service]
+Type=simple
+# EMERGENCY_NUMBERS only, and only so the oFono EmergencyNumbers property
+# matches imsd's configuration; imsd itself owns emergency classification.
+EnvironmentFile=-/etc/imsd.env
+ExecStart=/usr/bin/imsd-ofonod
+# it holds no call state of its own — imsd is re-queried on every reconnect
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target