nats: the module, and an image that reloads in place

Step 1.1 and 1.2 of novox/hq ADR 0116. The server is a built artifact rather
than the upstream image directly, because it needs an entrypoint of its own:
the host can only recreate a container, and recreating the bus for every
permission change drops every connection and every in-flight ack. nats-server
reloads on SIGHUP by itself, so the config is mounted as a directory (not
digest-tracked, hq issue 103) and the entrypoint watches the one file.

Verified against the real server, not assumed: a user added to the config
connects, a revoked one is refused, both within one poll interval, with the
container's PID and restart count unchanged and "Reloaded: accounts" in its
log.

Two corrections found by checking rather than reading:
- the seat delivers nothing now (hq ADR 0117), and the controller's parser
  refused the manifest until it did — "nats claims mesh-broker, whose holder
  answers for amqp, and nats does not provide amqp"
- pinned to the multi-arch index digest; the first pin was the amd64
  manifest, which builds here and fails on any other architecture

modules/nats/entrypoint.sh

@@ -0,0 +1,61 @@
+#!/bin/sh
+# nats's entrypoint: run the server, and reload it in place when the mesh rewrites its
+# configuration.
+#
+# **Why this exists inside the module** (novox/hq design 25 §5). The controller composes every
+# account and permission into one configuration file, and that file changes whenever a module is
+# added, reassigned, or a person's access is granted or revoked — which is often, and on the one
+# server everything else depends on. The host has no way to say "reload this container": a
+# container resource has `restart-on` and nothing else, and a container's `restart-on` means
+# *recreate* — every connection dropped and every in-flight JetStream ack lost, mid-flight, for a
+# permission change. `reload-on` is real but it is a *service* field, not a container's.
+#
+# nats-server already reloads its own configuration on SIGHUP — accounts, permissions, everything
+# the mesh composes — without dropping a connection. That is the server's own documented
+# capability, not something built for the mesh. So the configuration is mounted as a directory
+# (a directory's contents are not digest-tracked the way a directly-mounted file's are, novox/hq
+# issue 103), and this watches the one file inside it and signals the server itself. The host's
+# only job is what it already does for any directory: keep the file's content current. Nothing
+# here is declared `restart-on` or `reload-on`.
+set -eu
+
+CONF="${MESH_NATS_CONF:-/etc/nats/nats.conf}"
+POLL="${MESH_NATS_CONF_POLL_SECONDS:-5}"
+
+# The controller writes the configuration as part of the same declaration that creates this
+# container, but the two are not ordered against each other. Waiting is correct and starting
+# without one is not: nats-server would come up with its compiled-in defaults — no TLS, no
+# accounts, every subject open to anyone who can reach the port — and then be reloaded into
+# correctness a moment later. A bus that is briefly open to everything is not a bus that is
+# briefly wrong; it is an open bus.
+while [ ! -s "$CONF" ]; do
+  echo "[nats] waiting for the mesh to compose $CONF"
+  sleep 1
+done
+
+digest() { sha256sum "$CONF" 2>/dev/null | cut -d' ' -f1; }
+
+nats-server --config "$CONF" "$@" &
+server=$!
+
+# Forward a stop to the server and let it drain, rather than dying and leaving it orphaned as
+# PID 1's child.
+stop() { kill -TERM "$server" 2>/dev/null || true; }
+trap stop TERM INT
+
+last=$(digest)
+while kill -0 "$server" 2>/dev/null; do
+  sleep "$POLL"
+  now=$(digest)
+  # An empty digest means the file is mid-write or briefly gone. Reloading on that would hand the
+  # server a truncated configuration; the next tick sees the finished one.
+  [ -n "$now" ] || continue
+  if [ "$now" != "$last" ]; then
+    last=$now
+    echo "[nats] configuration changed; reloading in place"
+    kill -HUP "$server" || true
+  fi
+done
+
+# `wait` on an already-exited child still yields its status, which becomes this container's.
+wait "$server"