docs(dev): resolve DEBT-030 phase A + add mutator-family bus smoke

- scripts/bus/smoke-mutator.sh: boots decnet bus, subscribes to
  topology.>, publishes one event per mutation-lifecycle state plus
  a topology.status transition, asserts all four land on the
  subscriber. Cheap E2E for the topic hierarchy the mutator + SSE
  route rely on.
- development/DEBT.md: mark DEBT-030 ✅ resolved (Phase A) with a
  summary of what shipped; flag the optimistic staged-buffer editor
  as Phase B follow-up, not debt.

scripts/bus/smoke-mutator.sh

@@ -0,0 +1,90 @@
+#!/usr/bin/env bash
+# Mutator-family topic smoke test: boots a bus worker, subscribes to
+# `topology.>`, publishes one event per mutation-lifecycle state
+# (enqueued → applying → applied) plus a topology.status transition,
+# and verifies each lands on the subscriber.
+#
+# This is a cheap E2E for the topic hierarchy wired into the mutator
+# and SSE route — the full DB + mutator + API loop is exercised by the
+# pytest suite under tests/topology/ and tests/api/topology/.
+#
+# Usage: scripts/bus/smoke-mutator.sh
+set -euo pipefail
+
+SOCK="$(mktemp -u -t decnet-bus-mut-smoke.XXXXXX.sock)"
+export DECNET_BUS_SOCKET="${SOCK}"
+LOGDIR="$(mktemp -d -t decnet-bus-mut-smoke.XXXXXX)"
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+TID="smoke-$(date +%s)"
+
+cleanup() {
+    kill "${SUB_PID:-0}" 2>/dev/null || true
+    kill "${WORKER_PID:-0}" 2>/dev/null || true
+    wait 2>/dev/null || true
+    rm -f "${SOCK}"
+    rm -rf "${LOGDIR}"
+}
+trap cleanup EXIT
+
+echo "smoke-mutator: socket=${SOCK} topology=${TID}"
+
+decnet bus --socket "${SOCK}" --group "" --heartbeat 5 \
+    > "${LOGDIR}/worker.log" 2>&1 &
+WORKER_PID=$!
+
+for _ in {1..40}; do
+    [[ -S "${SOCK}" ]] && break
+    sleep 0.05
+done
+if [[ ! -S "${SOCK}" ]]; then
+    echo "smoke-mutator: FAIL — bus worker never created ${SOCK}" >&2
+    cat "${LOGDIR}/worker.log" >&2
+    exit 1
+fi
+
+python "${HERE}/sub.py" 'topology.>' > "${LOGDIR}/sub.log" 2>&1 &
+SUB_PID=$!
+
+sleep 0.3
+
+publish() {
+    local topic="$1" payload="$2"
+    python "${HERE}/pub.py" "${topic}" "${payload}" >/dev/null
+}
+
+publish "topology.${TID}.mutation.enqueued"  '{"mutation_id": "m1", "op": "add_lan"}'
+publish "topology.${TID}.mutation.applying"  '{"mutation_id": "m1", "op": "add_lan"}'
+publish "topology.${TID}.mutation.applied"   '{"mutation_id": "m1", "op": "add_lan"}'
+publish "topology.${TID}.status"             '{"state": "degraded", "reason": "smoke"}'
+
+expected=(
+    "topology.${TID}.mutation.enqueued"
+    "topology.${TID}.mutation.applying"
+    "topology.${TID}.mutation.applied"
+    "topology.${TID}.status"
+)
+
+for _ in {1..60}; do
+    missing=0
+    for topic in "${expected[@]}"; do
+        if ! grep -q "${topic}" "${LOGDIR}/sub.log"; then
+            missing=1
+            break
+        fi
+    done
+    [[ "${missing}" -eq 0 ]] && break
+    sleep 0.05
+done
+
+for topic in "${expected[@]}"; do
+    if ! grep -q "${topic}" "${LOGDIR}/sub.log"; then
+        echo "smoke-mutator: FAIL — missing ${topic}" >&2
+        echo "--- worker.log ---" >&2; cat "${LOGDIR}/worker.log" >&2
+        echo "--- sub.log ---"    >&2; cat "${LOGDIR}/sub.log"    >&2
+        exit 1
+    fi
+done
+
+echo "smoke-mutator: OK — all 4 mutator-family events delivered"
+grep -E 'mutation|status' "${LOGDIR}/sub.log" || true