feat(wake): W6 — off-host dead-man beacon + pluggable alarm-sink adapter (#910)

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

packages/mosaic/framework/tools/wake/detector.sh

@@ -402,6 +402,18 @@ cmd_run() {
     # A single failing source must not kill the long-lived service; poll-once
     # already failed loud on stderr for it. The loop keeps serving healthy ones.
     cmd_poll_once || true
+    # W6 SEAM (§1.3, off-host dead-man beacon): emit ONE monotonic liveness
+    # beacon per poll cycle. This is the ONLY W6 call site in the detector — a
+    # single, clean, opt-in seam. It is INERT unless the operator wires a beacon
+    # sink (WAKE_BEACON_SINK_CMD); wiring + install-validating that target is W7's
+    # job, not the detector's. Liveness is SPLIT from work-triggering, so a beacon
+    # emit failure NEVER kills the detector loop — but it is loud (not silent),
+    # and the off-host monitor's absence check (beacon.sh check) is the real
+    # safety net regardless of what this dying host does.
+    if [ -n "${WAKE_BEACON_SINK_CMD:-}" ]; then
+      "$SCRIPT_DIR/beacon.sh" emit >/dev/null || \
+        echo "detector.sh: WARN — off-host liveness beacon emit failed (see beacon.sh); the off-host absence check remains the authoritative dead-man." >&2
+    fi
     [ "$once" -eq 1 ] && break
     sleep "$interval"
   done