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.

docker/Dockerfile.ohm

@@ -0,0 +1,66 @@
+# Minimal cross-build of shairport-sync with the OHM Songcast sender backend,
+# for linux/arm64 (Raspberry Pi OS aarch64). Produces a glibc binary compatible
+# with Debian bullseye (glibc 2.31) and later (e.g. bookworm).
+#
+# Usage from the repo root:
+#   docker buildx build --platform linux/arm64 --target binary \
+#     -f docker/Dockerfile.ohm --output type=local,dest=./out-ohm .
+#
+# Then copy ./out-ohm/shairport-sync to the Pi.
+#
+# FFmpeg is taken from Debian packages (libavcodec-dev etc.) rather than built
+# from source, to keep the cross-build fast under emulation.
+
+ARG DEBIAN_VERSION=bullseye
+
+FROM --platform=linux/arm64 debian:${DEBIAN_VERSION} AS builder
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        build-essential \
+        autoconf \
+        automake \
+        libtool \
+        pkg-config \
+        git \
+        ca-certificates \
+        xxd \
+        libasound2-dev \
+        libplist-dev \
+        libplist-utils \
+        libssl-dev \
+        libsodium-dev \
+        libgcrypt-dev \
+        uuid-dev \
+        libavcodec-dev \
+        libavformat-dev \
+        libswresample-dev \
+        libavutil-dev \
+        libavahi-client-dev \
+        avahi-daemon \
+        libdbus-1-dev \
+        libglib2.0-dev \
+        libconfig-dev \
+        libpopt-dev \
+        libsndfile-dev \
+        libsoxr-dev \
+        xmltoman \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /shairport-sync
+COPY . .
+
+WORKDIR /shairport-sync/build
+RUN autoreconf -i ../
+RUN ../configure --sysconfdir=/etc --with-alsa \
+        --with-avahi --with-ssl=openssl \
+        --with-airplay-2 --with-ffmpeg --with-metadata \
+        --with-ohm --with-dummy --with-pipe --with-stdout \
+        --with-dbus-interface --with-mpris-interface \
+        --with-convolution --with-soxr
+RUN make -j"$(nproc)"
+
+# Target that yields just the binary on the host filesystem via --output.
+FROM --platform=linux/arm64 scratch AS binary
+COPY --from=builder /shairport-sync/build/shairport-sync /shairport-sync