Add ohm, an OpenHome Songcast sender audio backend

Streams AirPlay audio to OpenHome/Songcast receivers as OHU unicast PCM
(16/24-bit). Sends are paced on the PTP master clock so receivers slave
to the AirPlay group clock through their buffer-fill servo; MediaLatency
carries the phase, matching the wire behaviour of Linn's own sender
(zero flags and timestamps). delay() reports the newest frame's playtime
lead so shairport feeds a receiver latency ahead. Handles Join, Listen,
Leave and Resend with a frame history, and forwards track metadata and
metatext from the metadata hub.

With -r <name>, ohm_upnp controls the receiver over UPnP: discovered by
friendly name via SSDP at time of use, woken, switched to its Songcast
source and pointed at this sender when a session starts; volume follows
the AirPlay slider capped at the device's VolumeLimit; standby follows
some minutes after the session ends.

ohm_upnp.h

@@ -0,0 +1,58 @@
+/*
+ * OpenHome receiver control for the ohm audio backend.
+ *
+ * This file is part of Shairport Sync.
+ * Copyright (c) Nils Schneider 2026
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#pragma once
+
+#include <stddef.h>
+
+/* XML-escape src into dst (NUL-terminated). Returns chars written (excluding NUL). */
+size_t ohm_xml_escape(char *dst, size_t dst_cap, const char *src);
+
+/* The backend declares volume as attenuation -OHM_UPNP_VOLUME_RANGE_CDB..0
+ * (dB * 100); shairport stretches the AirPlay slider over exactly this range. */
+#define OHM_UPNP_VOLUME_RANGE_CDB 6000
+
+typedef struct {
+  const char *receiver_name; /* friendly name to match; NULL disables the module */
+  const char *interface_ip;
+  const char *display_name;
+  int standby_minutes; /* 0 = never */
+  int ohu_port;
+} ohm_upnp_config_t;
+
+/* Strings are copied. */
+void ohm_upnp_configure(const ohm_upnp_config_t *config);
+int ohm_upnp_enabled(void);
+
+/* Callback side: cheap, thread-safe, never block. */
+void ohm_upnp_session_begin(void);
+void ohm_upnp_session_end(void);
+void ohm_upnp_set_volume(long vol_cdb); /* dB * 100, <= 0 */
+
+/* Does the network work (SSDP discovery, SOAP); call periodically from one
+ * thread. */
+void ohm_upnp_tick(void);
+
+void ohm_upnp_deinit(void);