feat(869-c3): lease-broker supervisor unit (Part of #869)

Part of #869

Mos (id-11) Gate-16 merge: independent APPROVE @75235ef8 (9/9, no live host mutation), author id2 != approver id11, CI green wp1971.

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/framework/systemd/user/mosaic-lease-broker.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Mosaic lease broker daemon (framework tools/lease-broker/daemon.py)
+Documentation=https://git.mosaicstack.dev/mosaicstack/stack
+After=default.target
+
+[Service]
+Type=simple
+# The broker socket lives under the runtime directory so it disappears with
+# the user session instead of surviving as stale state across logins.
+# daemon.py's secure_parent() fails closed unless this directory is exactly
+# 0700, so RuntimeDirectoryMode is not cosmetic.
+RuntimeDirectory=mosaic-lease
+RuntimeDirectoryMode=0700
+# Remove loader and noninteractive-shell controls before ExecStart loads env,
+# matching the tmux fleet units in this same directory.
+UnsetEnvironment=LD_PRELOAD BASH_ENV ENV
+ExecStart=/usr/bin/env -i HOME=%h PATH=/usr/bin:/bin XDG_RUNTIME_DIR=%t /bin/bash --noprofile --norc %h/.config/mosaic/tools/lease-broker/start-lease-broker.sh
+Restart=on-failure
+RestartSec=1
+
+[Install]
+WantedBy=default.target

packages/mosaic/framework/tools/lease-broker/start-lease-broker.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Supervisor entry point for the Mosaic lease broker daemon (issue #869, C3).
+#
+# Resolves the broker socket path with the SAME precedence as
+# `defaultLeaseBrokerSocket` in `packages/mosaic/src/commands/launch.ts`, so a
+# gated runtime launched through that client always finds the socket this
+# supervisor creates:
+#   1. an explicit MOSAIC_LEASE_BROKER_SOCKET
+#   2. "$XDG_RUNTIME_DIR/mosaic-lease/broker.sock"
+#   3. "/run/user/<uid>/mosaic-lease/broker.sock"
+#
+# The state file is colocated next to the socket (same directory,
+# "state.json"), mirroring how the broker already colocates its per-session
+# generation files beside the socket.
+#
+# This script never installs, enables, or starts the systemd unit that calls
+# it; it is only ever invoked BY that unit (or by a human/test harness that
+# passes its own HOME/XDG_RUNTIME_DIR).
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+
+if [ -n "${MOSAIC_LEASE_BROKER_SOCKET:-}" ]; then
+  SOCKET="$MOSAIC_LEASE_BROKER_SOCKET"
+else
+  RUNTIME_DIR="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+  SOCKET="$RUNTIME_DIR/mosaic-lease/broker.sock"
+fi
+STATE="$(dirname -- "$SOCKET")/state.json"
+
+exec python3 "$SCRIPT_DIR/daemon.py" --socket "$SOCKET" --state "$STATE"