feat(wallet): Ark protocol support via barkd sidecar

barkd (Ark wallet daemon, pinned 0.3.0, checksum-verified release binary)
packaged as an installable app; thin HTTP bridge in wallet/ark_client.rs
mirrors the fedimint_client pattern — the bark SDK stays out of the node
binary. wallet.ark-* RPCs cover status/balance/address/send/invoice/
board/offboard/history/configure; Ark movements merge into the unified
ecash history (kind="ark") and spendable Ark sats into total_sats.
Signet defaults (Second's public Ark server) until Ark matures.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/barkd/Dockerfile

@@ -0,0 +1,32 @@
+# barkd — Ark protocol wallet daemon (https://gitlab.com/ark-bitcoin/bark).
+# No official upstream image exists (their GitLab registry is empty), so we
+# package the pinned, checksum-verified release binary ourselves and push to
+# the node registry — same approach as fmcd. Keep the version in lockstep with
+# the REST shapes coded in core/archipelago/src/wallet/ark_client.rs (0.3.0).
+FROM debian:bookworm-slim
+
+ARG BARKD_VERSION=0.3.0
+ARG BARKD_SHA256=8562fa27386bae666ed62fa95c92d40f7bdb20d22525f75799adfc16adaaedb3
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends ca-certificates curl && \
+    curl -fsSL "https://gitlab.com/api/v4/projects/ark-bitcoin%2Fbark/packages/generic/release-assets/bark-${BARKD_VERSION}/barkd-${BARKD_VERSION}-linux-x86_64" \
+        -o /usr/local/bin/barkd && \
+    echo "${BARKD_SHA256}  /usr/local/bin/barkd" | sha256sum -c - && \
+    chmod a+x /usr/local/bin/barkd && \
+    apt-get purge -y curl && apt-get autoremove -y && \
+    apt-get clean && rm -rf /var/lib/apt/lists/*
+
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod a+x /entrypoint.sh
+
+# The wallet itself is created over REST by the node's Ark bridge
+# (wallet.ark-* RPCs) — the container just runs the daemon.
+ENV BARKD_DATADIR=/data \
+    BARKD_BIND_HOST=0.0.0.0 \
+    BARKD_BIND_PORT=3535
+
+EXPOSE 3535
+VOLUME /data
+
+ENTRYPOINT ["/entrypoint.sh"]