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the VoiceCallManager interface is withdrawn and re-added, Calls keeps the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/APKBUILD

@@ -19,6 +19,11 @@ makedepends="clang lld libc++-dev llvm-libunwind-dev llvm-runtimes glib-dev pkgc
 # requests, which races imsd for the PDN and flaps it with a new prefix every
 # ~2.5 min — the two IMS stacks cannot share one PDN
 provides="81voltd=$pkgver-r$pkgrel"
+# imsd-ofono is opt-in: it carries the org.ofono shim that lets GNOME Calls
+# (Phosh, GNOME Mobile) drive imsd, plus the gschema override that repoints
+# Calls at the ofono provider. A Plasma Mobile phone wants imsd-dialerd, which
+# is in the main package, and must NOT get that override.
+subpackages="$pkgname-ofono:ofono"
 source="$pkgname-$pkgver.tar.gz::$url/archive/v$pkgver.tar.gz"
 builddir="$srcdir/$pkgname"
 
@@ -33,3 +38,12 @@ check() {
 package() {
 	make install DESTDIR="$pkgdir"
 }
+
+ofono() {
+	pkgdesc="GNOME Calls (Phosh, GNOME Mobile) backend for imsd"
+	depends="$pkgname=$pkgver-r$pkgrel calls"
+	amove usr/bin/imsd-ofonod
+	amove usr/lib/systemd/system/imsd-ofonod.service
+	amove usr/share/dbus-1/system.d/imsd-ofono.conf
+	amove usr/share/glib-2.0/schemas/90_imsd-ofono.gschema.override
+}